The general orientation of our team is described by the short name given to it: Special Functions, that is, particular mathematical functions that have established names due to their importance in mathematical analysis, physics, and other application domains. Indeed, we ambition to study special functions with the computer, by combined means of computer algebra and formal methods.
Computer-algebra systems have been advertised for decades as software for “doing mathematics by computer” . For instance, computer-algebra libraries can uniformly generate a corpus of mathematical properties about special functions, so as to display them on an interactive website. This possibility was recently shown by the computer-algebra component of the team . Such an automated generation significantly increases the reliability of the mathematical corpus, in comparison to the content of existing static authoritative handbooks. The importance of the validity of these contents can be measured by the very wide audience that such handbooks have had, to the point that a book like remains one of the most cited mathematical publications ever and has motivated the 10-year-long project of writing its successor . However, can the mathematics produced “by computer” be considered as true mathematics? More specifically, whereas it is nowadays well established that the computer helps in discovering and observing new mathematical phenomenons, can the mathematical statements produced with the aid of the computer and the mathematical results computed by it be accepted as valid mathematics, that is, as having the status of mathematical proofs? Beyond the reported weaknesses or controversial design choices of mainstream computer-algebra systems, the issue is more of an epistemological nature. It will not find its solution even in the advent of the ultimate computer-algebra system: the social process of peer-reviewing just falls short of evaluating the results produced by computers, as reported by Th. Hales after the publication of his proof of the Kepler Conjecture about sphere packing.
A natural answer to this deadlock is to move to an alternative kind of
mathematical software and to use a proof assistant to check the
correctness of the desired properties or formulas. The success
of large-scale formalization projects, like the Four-Color Theorem of
graph theory , the above-mentioned Kepler
Conjecture , and the Odd Order
Theorem of group theory
The Dynamic Dictionary of Mathematical Functions
The formal-proofs component of the team emanates from another project
of the MSR–Inria Joint Centre, namely the Mathematical Components
project (MathComp)
The present team takes benefit from these recent advances to explore the formal certification of the results collected in DDMF. The aim of this project is to concentrate the formalization effort on this delimited area, building on DDMF and the Algolib library, as well as on the Coq system and on the libraries developed by the MathComp project.
The following few opinions on computer algebra are, we believe, typical of computer-algebra users' doubts and difficulties when using computer-algebra systems:
Fredrik Johansson, expert in the multi-precision numerical evaluation of special functions and in fast computer-algebra algorithms, writes on his blog : “Mathematica is great for cross-checking numerical values, but it's not unusual to run into bugs, so triple checking is a good habit.” One answer in the discussion is: “We can claim that Mathematica has [...] an impossible to understand semantics: If Mathematica's output is wrong then change the input. If you don't like the answer, change the question. That seems to be the philosophy behind.”
A professor's advice to students on using Maple: “You may wish to use Maple to check your homework answers. If you do then keep in mind that Maple sometimes gives the wrong answer, usually because you asked incorrectly, or because of niceties of analytic continuation. You may even be bitten by an occasional Maple bug, though that has become fairly unlikely. Even with as powerful a tool as Maple you will still have to devise your own checks and you will still have to think.”
Jacques Carette, former head of the maths group at Maplesoft, about a bug when asking Maple to take the limit limit(f(n) * exp(-n), n = infinity) for an undetermined function f: “The problem is that there is an implicit assumption in the implementation that unknown functions do not `grow too fast'.”
As explained by the expert views above, complaints by computer-algebra users are often due to their misunderstanding of what a computer-algebra systems is, namely a purely syntactic tool for calculations, that the user must complement with a semantics. Still, robustness and consistency of computer-algebra systems are not ensured as of today, and, whatever Zeilberger may provocatively say in his Opinion 94 , a firmer logical foundation is necessary. Indeed, the fact is that many “bugs” in a computer-algebra system cannot be fixed by just the usual debugging method of tracking down the faulty lines in the code. It is sort of “by design”: assumptions that too often remain implicit are really needed by the design of symbolic algorithms and cannot easily be expressed in the programming languages used in computer algebra. A similar certification initiative has already been undertaken in the domain of numerical computing, in a successful manner , . It is natural to undertake a similar approach for computer algebra.
Some of the mathematical objects that interest our team are still totally untouched by formalization. When implementing them and their theory inside a proof assistant, we have to deal with the pervasive discrepancy between the published literature and the actual implementation of computer-algebra algorithms. Interestingly, this forces us to clarify our computer-algebraic view on them, and possibly make us discover holes lurking in published (human) proofs. We are therefore convinced that the close interaction of researchers from both fields, which is what we strive to maintain in this team, is a strong asset.
For a concrete example, the core of Zeilberger's creative telescoping manipulates rational functions up to simplifications. In summation applications, checking that these simplifications do not hide problematic divisions by 0 is most often left to the reader. In the same vein, in the case of integrals, the published algorithms do not check the convergence of all integrals, especially in intermediate calculations. Such checks are again left to the readers. In general, we expect to revisit the existing algorithms to ensure that they are meaningful for genuine mathematical sequences or functions, and not only for algebraic idealizations.
Another big challenge in this project originates in the scientific difference between computer algebra and formal proofs. Computer algebra seeks speed of calculation on concrete instances of algebraic data structures (polynomials, matrices, etc). For their part, formal proofs manipulate symbolic expressions in terms of abstract variables understood to represent generic elements of algebraic data structures. In view of this, a continuous challenge is to develop the right, hybrid thinking attitude that is able to effectively manage concrete and abstract values simultaneously, alternatively computing and proving with them.
Applications in combinatorics and mathematical physics frequently involve equations of so high orders and so large sizes, that computing or even storing all their coefficients is impossible on existing computers. Making this tractable is an extraordinary challenge. The approach we believe in is to design algorithms of good—ideally quasi-optimal—complexity in order to extract precisely the required data from the equations, while avoiding the computationally intractable task of completely expanding them into an explicit representation.
Typical applications with expected high impact are the automatic discovery and algorithmic proof of results in combinatorics and mathematical physics for which human proofs are currently unattainable.
The implementation of certified symbolic computations on special functions in the Coq proof assistant requires both investigating new formalization techniques and renewing the traditional computer-algebra viewpoint on these standard objects. Large mathematical objects typical of computer algebra occur during formalization, which also requires us to improve the efficiency and ergonomics of Coq. In order to feed this interdisciplinary activity with new motivating problems, we additionally pursue a research activity oriented towards experimental mathematics in application domains that involve special functions. We expect these applications to pose new algorithmic challenges to computer algebra, which in turn will deserve a formal-certification effort. Finally, DDMF is the motivation and the showcase of our progress on the certification of these computations. While striving to provide a formal guarantee of the correctness of the information it displays, we remain keen on enriching its mathematical content by developing new computer-algebra algorithms.
Our formalization effort consists in organizing a cooperation between a computer-algebra system and a proof assistant. The computer-algebra system is used to produce efficiently algebraic data, which are later processed by the proof assistant. The success of this cooperation relies on the design of appropriate libraries of formalized mathematics, including certified implementations of certain computer-algebra algorithms. On the other side, we expect that scrutinizing the implementation and the output of computer-algebra algorithms will shed a new light on their semantics and on their correctness proofs, and help clarifying their documentation.
The appropriate framework for the study of efficient algorithms for special functions is algebraic. Representing algebraic theories as Coq formal libraries takes benefit from the methodology emerging from the success of ambitious projects like the formal proof of a major classification result in finite-group theory (the Odd Order Theorem) .
Yet, a number of the objects we need to formalize in the present context has never been investigated using any interactive proof assistant, despite being considered as commonplaces in computer algebra. For instance there is up to our knowledge no available formalization of the theory of non-commutative rings, of the algorithmic theory of special-functions closures, or of the asymptotic study of special functions. We expect our future formal libraries to prove broadly reusable in later formalizations of seemingly unrelated theories.
Another peculiarity of the mathematical objects we are going to manipulate with the Coq system is their size. In order to provide a formal guarantee on the data displayed by DDMF, two related axes of research have to be pursued. First, efficient algorithms dealing with these large objects have to be programmed and run in Coq. Recent evolutions of the Coq system to improve the efficiency of its internal computations , make this objective reachable. Still, how to combine the aforementioned formalization methodology with these cutting-edge evolutions of Coq remains one of the prospective aspects of our project. A second need is to help users interactively manipulate large expressions occurring in their conjectures, an objective for which little has been done so far. To address this need, we work on improving the ergonomics of the system in two ways: first, ameliorating the reactivity of Coq in its interaction with the user; second, designing and implementing extensions of its interface to ease our formalization activity. We expect the outcome of these lines of research to be useful to a wider audience, interested in manipulating large formulas on topics possibly unrelated to special functions.
Our algorithm certifications inside Coq intend to simulate well-identified components of our Maple packages, possibly by reproducing them in Coq. It would however not have been judicious to re-implement them inside Coq in a systematic way. Indeed for a number of its components, the output of the algorithm is more easily checked than found, like for instance the solving of a linear system. Rather, we delegate the discovery of the solutions to an external, untrusted oracle like Maple. Trusted computations inside Coq then formally validate the correctness of the a priori untrusted output. More often than not, this validation consists in implementing and executing normalization procedures inside Coq. A challenge of this automation is to make sure they go to scale while remaining efficient, which requires a Coq version of non-trivial computer-algebra algorithms. A first, archetypal example we expect to work on is a non-commutative generalization of the normalization procedure for elements of rings .
Generally speaking, we design algorithms for manipulating special functions symbolically, whether univariate or with parameters, and for extracting algorithmically any kind of algebraic and analytic information from them, notably asymptotic properties. Beyond this, the heart of our research is concerned with parametrised definite summations and integrations. These very expressive operations have far-ranging applications, for instance, to the computation of integral transforms (Laplace, Fourier) or to the solution of combinatorial problems expressed via integrals (coefficient extractions, diagonals). The algorithms that we design for them need to really operate on the level of linear functional systems, differential and of recurrence. In all cases, we strive to design our algorithms with the constant goal of good theoretical complexity, and we observe that our algorithms are also fast in practice.
Our long-term goal is to design fast algorithms for a general method for special-function integration (creative telescoping), and make them applicable to general special-function inputs. Still, our strategy is to proceed with simpler, more specific classes first (rational functions, then algebraic functions, hyperexponential functions, D-finite functions, non-D-finite functions; two variables, then many variables); as well, we isolate analytic questions by first considering types of integration with a more purely algebraic flavor (constant terms, algebraic residues, diagonals of combinatorics). In particular, we expect to extend our recent approach to more general classes (algebraic with nested radicals, for example): the idea is to speed up calculations by making use of an analogue of Hermite reduction that avoids considering certificates. Homologous problems for summation will be addressed as well.
As a consequence of our complexity-driven approach to algorithms design, the algorithms mentioned in the previous paragraph are of good complexity. Therefore, they naturally help us deal with applications that involve equations of high orders and large sizes.
With regard to combinatorics, we expect to advance the algorithmic classification of combinatorial classes like walks and urns. Here, the goal is to determine if enumerative generating functions are rational, algebraic, or D-finite, for example. Physical problems whose modelling involves special-function integrals comprise the study of models of statistical mechanics, like the Ising model for ferro-magnetism, or questions related to Hamiltonian systems.
Number theory is another promising domain of applications. Here, we attempt an experimental approach to the automated certification of integrality of the coefficients of mirror maps for Calabi–Yau manifolds. This could also involve the discovery of new Calabi–Yau operators and the certification of the existing ones. We also plan to algorithmically discover and certify new recurrences yielding good approximants needed in irrationality proofs.
It is to be noted that in all of these application domains, we would so far use general algorithms, as was done in earlier works of ours , , . To push the scale of applications further, we plan to consider in each case the specifics of the application domain to tailor our algorithms.
In continuation of our past project of an encyclopedia at
http://
the algorithmic discussion of equations with parameters, leading to certified automatic case analysis based on arithmetic properties of the parameters;
lists of summation and integral formulas involving special functions, including validity conditions on the parameters;
guaranteed large-precision numerical evaluations.
Computer algebra manipulates symbolic representations of exact mathematical objects in a computer, in order to perform computations and operations like simplifying expressions and solving equations for “closed-form expressions”. The manipulations are often fundamentally of algebraic nature, even when the ultimate goal is analytic. The issue of efficiency is a particular one in computer algebra, owing to the extreme swell of the intermediate values during calculations.
Our view on the domain is that research on the algorithmic manipulation of special functions is anchored between two paradigms:
adopting linear differential equations as the right data structure for special functions,
designing efficient algorithms in a complexity-driven way.
It aims at four kinds of algorithmic goals:
algorithms combining functions,
functional equations solving,
multi-precision numerical evaluations,
guessing heuristics.
This interacts with three domains of research:
computer algebra, meant as the search for quasi-optimal algorithms for exact algebraic objects,
symbolic analysis/algebraic analysis;
experimental mathematics (combinatorics, mathematical physics, ...).
This view is made explicit in the present section.
Numerous special functions satisfy linear differential and/or recurrence equations. Under a mild technical condition, the existence of such equations induces a finiteness property that makes the main properties of the functions decidable. We thus speak of D-finite functions. For example, 60 % of the chapters in the handbook describe D-finite functions. In addition, the class is closed under a rich set of algebraic operations. This makes linear functional equations just the right data structure to encode and manipulate special functions. The power of this representation was observed in the early 1990s , leading to the design of many algorithms in computer algebra. Both on the theoretical and algorithmic sides, the study of D-finite functions shares much with neighbouring mathematical domains: differential algebra, D-module theory, differential Galois theory, as well as their counterparts for recurrence equations.
Differential/recurrence equations that define special functions can be recombined to define: additions and products of special functions; compositions of special functions; integrals and sums involving special functions. Zeilberger's fast algorithm for obtaining recurrences satisfied by parametrised binomial sums was developed in the early 1990s already . It is the basis of all modern definite summation and integration algorithms. The theory was made fully rigorous and algorithmic in later works, mostly by a group in Risc (Linz, Austria) and by members of the team , , , , , . The past ÉPI Algorithms contributed several implementations (gfun , Mgfun ).
Encoding special functions as defining linear functional equations postpones some of the difficulty of the problems to a delayed solving of equations. But at the same time, solving (for special classes of functions) is a sub-task of many algorithms on special functions, especially so when solving in terms of polynomial or rational functions. A lot of work has been done in this direction in the 1990s; more intensively since the 2000s, solving differential and recurrence equations in terms of special functions has also been investigated.
A major conceptual and algorithmic difference exists for numerical calculations between data structures that fit on a machine word and data structures of arbitrary length, that is, multi-precision arithmetic. When multi-precision floating-point numbers became available, early works on the evaluation of special functions were just promising that “most” digits in the output were correct, and performed by heuristically increasing precision during intermediate calculations, without intended rigour. The original theory has evolved in a twofold way since the 1990s: by making computable all constants hidden in asymptotic approximations, it became possible to guarantee a prescribed absolute precision; by employing state-of-the-art algorithms on polynomials, matrices, etc, it became possible to have evaluation algorithms in a time complexity that is linear in the output size, with a constant that is not more than a few units. On the implementation side, several original works exist, one of which (NumGfun ) is used in our DDMF.
“Differential approximation”, or “Guessing”, is an operation to get an ODE likely to be satisfied by a given approximate series expansion of an unknown function. This has been used at least since the 1970s and is a key stone in spectacular applications in experimental mathematics . All this is based on subtle algorithms for Hermite–Padé approximants . Moreover, guessing can at times be complemented by proven quantitative results that turn the heuristics into an algorithm . This is a promising algorithmic approach that deserves more attention than it has received so far.
The main concern of computer algebra has long been to prove the feasibility of a given problem, that is, to show the existence of an algorithmic solution for it. However, with the advent of faster and faster computers, complexity results have ceased to be of theoretical interest only. Nowadays, a large track of works in computer algebra is interested in developing fast algorithms, with time complexity as close as possible to linear in their output size. After most of the more pervasive objects like integers, polynomials, and matrices have been endowed with fast algorithms for the main operations on them , the community, including ourselves, started to turn its attention to differential and recurrence objects in the 2000s. The subject is still not as developed as in the commutative case, and a major challenge remains to understand the combinatorics behind summation and integration. On the methodological side, several paradigms occur repeatedly in fast algorithms: “divide and conquer” to balance calculations, “evaluation and interpolation” to avoid intermediate swell of data, etc. .
Handbooks collecting mathematical properties aim at serving as
reference, therefore trusted, documents. The decision of
several authors or maintainers of such knowledge bases to move from paper
books , , to websites and wikis
Several attempts have been made in order to extend existing computer-algebra systems with symbolic manipulations of logical formulas. Yet, these works are more about extending the expressivity of computer-algebra systems than about improving the standards of correctness and semantics of the systems. Conversely, several projects have addressed the communication of a proof system with a computer-algebra system, resulting in an increased automation available in the proof system, to the price of the uncertainty of the computations performed by this oracle.
More ambitious projects have tried to design a new computer-algebra system providing an environment where the user could both program efficiently and elaborate formal and machine-checked proofs of correctness, by calling a general-purpose proof assistant like the Coq system. This approach requires a huge manpower and a daunting effort in order to re-implement a complete computer-algebra system, as well as the libraries of formal mathematics required by such formal proofs.
The move to machine-checked proofs of the mathematical correctness of the output of computer-algebra implementations demands a prior clarification about the often implicit assumptions on which the presumably correctly implemented algorithms rely. Interestingly, this preliminary work, which could be considered as independent from a formal certification project, is seldom precise or even available in the literature.
A number of authors have investigated ways to organize the communication of a chosen computer-algebra system with a chosen proof assistant in order to certify specific components of the computer-algebra systems, experimenting various combinations of systems and various formats for mathematical exchanges. Another line of research consists in the implementation and certification of computer-algebra algorithms inside the logic , , or as a proof-automation strategy. Normalization algorithms are of special interest when they allow to check results possibly obtained by an external computer-algebra oracle . A discussion about the systematic separation of the search for a solution and the checking of the solution is already clearly outlined in .
Significant progress has been made in the certification of numerical applications by formal proofs. Libraries formalizing and implementing floating-point arithmetic as well as large numbers and arbitrary-precision arithmetic are available. These libraries are used to certify floating-point programs, implementations of mathematical functions and for applications like hybrid systems.
To be checked by a machine, a proof needs to be expressed in a constrained, relatively simple formal language. Proof assistants provide facilities to write proofs in such languages. But, as merely writing, even in a formal language, does not constitute a formal proof just per se, proof assistants also provide a proof checker: a small and well-understood piece of software in charge of verifying the correctness of arbitrarily large proofs. The gap between the low-level formal language a machine can check and the sophistication of an average page of mathematics is conspicuous and unavoidable. Proof assistants try to bridge this gap by offering facilities, like notations or automation, to support convenient formalization methodologies. Indeed, many aspects, from the logical foundation to the user interface, play an important role in the feasibility of formalized mathematics inside a proof assistant.
While many logical foundations for mathematics have been proposed, studied, and implemented, type theory is the one that has been more successfully employed to formalize mathematics, to the notable exception of the Mizar system , which is based on set theory. In particular, the calculus of construction (CoC) and its extension with inductive types (CIC) , have been studied for more than 20 years and been implemented by several independent tools (like Lego, Matita, and Agda). Its reference implementation, Coq , has been used for several large-scale formalizations projects (formal certification of a compiler back-end; four-color theorem). Improving the type theory underlying the Coq system remains an active area of research. Other systems based on different type theories do exist and, whilst being more oriented toward software verification, have been also used to verify results of mainstream mathematics (prime-number theorem; Kepler conjecture).
The most distinguishing feature of CoC is that computation is promoted to the status of rigorous logical argument. Moreover, in its extension CIC, we can recognize the key ingredients of a functional programming language like inductive types, pattern matching, and recursive functions. Indeed, one can program effectively inside tools based on CIC like Coq. This possibility has paved the way to many effective formalization techniques that were essential to the most impressive formalizations made in CIC.
Another milestone in the promotion of the computations-as-proofs feature of Coq has been the integration of compilation techniques in the system to speed up evaluation. Coq can now run realistic programs in the logic, and hence easily incorporates calculations into proofs that demand heavy computational steps.
Because of their different choice for the underlying logic, other proof assistants have to simulate computations outside the formal system, and indeed fewer attempts to formalize mathematical proofs involving heavy calculations have been made in these tools. The only notable exception, which was finished in 2014, the Kepler conjecture, required a significant work to optimize the rewriting engine that simulates evaluation in Isabelle/HOL.
Programs run and proved correct inside the logic are especially useful for the conception of automated decision procedures. To this end, inductive types are used as an internal language for the description of mathematical objects by their syntax, thus enabling programs to reason and compute by case analysis and recursion on symbolic expressions.
The output of complex and optimized programs external to the proof assistant can also be stamped with a formal proof of correctness when their result is easier to check than to find. In that case one can benefit from their efficiency without compromising the level of confidence on their output at the price of writing and certify a checker inside the logic. This approach, which has been successfully used in various contexts, is very relevant to the present research project.
Representing abstract algebra in a proof assistant has been studied for long. The libraries developed by the MathComp project for the proof of the Odd Order Theorem provide a rather comprehensive hierarchy of structures; however, they originally feature a large number of instances of structures that they need to organize. On the methodological side, this hierarchy is an incarnation of an original work based on various mechanisms, primarily type inference, typically employed in the area of programming languages. A large amount of information that is implicit in handwritten proofs, and that must become explicit at formalization time, can be systematically recovered following this methodology.
Small-scale reflection
is another methodology promoted by the MathComp project.
Its ultimate goal is to ease formal proofs by systematically
dealing with as many bureaucratic steps as possible,
by automated computation.
For instance, as opposed to the style advocated by Coq's standard
library, decidable predicates are systematically represented
using computable boolean functions: comparison on integers
is expressed as program, and to state that
The MathComp library was consistently designed after uniform principles of software engineering. These principles range from simple ones, like naming conventions, to more advanced ones, like generic programming, resulting in a robust and reusable collection of formal mathematical components. This large body of formalized mathematics covers a broad panel of algebraic theories, including of course advanced topics of finite group theory, but also linear algebra, commutative algebra, Galois theory, and representation theory. We refer the interested reader to the online documentation of these libraries , which represent about 150,000 lines of code and include roughly 4,000 definitions and 13,000 theorems.
Topics not addressed by these libraries and that might be relevant to the present project include real analysis and differential equations. The most advanced work of formalization on these domains is available in the HOL-Light system , , , although some existing developments of interest , are also available for Coq. Another aspect of the MathComp libraries that needs improvement, owing to the size of the data we manipulate, is the connection with efficient data structures and implementations, which only starts to be explored.
The user of a proof assistant describes the proof he wants to formalize in the system using a textual language. Depending on the peculiarities of the formal system and the applicative domain, different proof languages have been developed. Some proof assistants promote the use of a declarative language, when the Coq and Matita systems are more oriented toward a procedural style.
The development of the large, consistent body of MathComp libraries has prompted the need to design an alternative and coherent language extension for the Coq proof assistant , , enforcing the robustness of proof scripts to the numerous changes induced by code refactoring and enhancing the support for the methodology of small-scale reflection.
The development of large libraries is quite a novelty for the Coq system. In particular any long-term development process requires the iteration of many refactoring steps and very little support is provided by most proof assistants, with the notable exception of Mizar . For the Coq system, this is an active area of research.
Pierre Lairez was awarded the SIAM/AAG (SIAM Activity Group on Algebraic Geometry) Early Career Prize.
Dynamic Mathematics on the Web
Functional Description: Programming tool for controlling the generation of mathematical websites that embed dynamical mathematical contents generated by computer-algebra calculations. Implemented in OCaml.
Participants: Alexis Darrasse, Frédéric Chyzak and Maxence Guesdon
Contact: Frédéric Chyzak
Encyclopedia of Combinatorial Structures
Functional Description: On-line mathematical encyclopedia with an emphasis on sequences that arise in the context of decomposable combinatorial structures, with the possibility to search by the first terms in the sequence, keyword, generating function, or closed form.
Participants: Alexis Darrasse, Frédéric Chyzak, Maxence Guesdon and Stéphanie Petit
Contact: Frédéric Chyzak
URL: http://
Dynamic Dictionary of Mathematical Functions
Functional Description: Web site consisting of interactive tables of mathematical formulas on elementary and special functions. The formulas are automatically generated by OCaml and computer-algebra routines. Users can ask for more terms of the expansions, more digits of the numerical values, proofs of some of the formulas, etc.
Participants: Alexandre Benoit, Alexis Darrasse, Bruno Salvy, Christoph Koutschan, Frédéric Chyzak, Marc Mezzarobba, Maxence Guesdon, Stefan Gerhold and Thomas Gregoire
Contact: Frédéric Chyzak
multivariate generating functions package
Functional Description: The Mgfun Project is a collection of packages for the computer algebra system Maple, and is intended for the symbolic manipulation of a large class of special functions and combinatorial sequences (in one or several variables and indices) that appear in many branches of mathematics, mathematical physics, and engineering sciences. Members of the class satisfy a crucial finiteness property which makes the class amenable to computer algebra methods and enjoy numerous algorithmic closure properties, including algorithmic closures under integration and summation.
Contact: Frédéric Chyzak
Functional Description: Ssreflect is a tactic language extension to the Coq system, developed by the Mathematical Components team.
Participants: Assia Mahboubi, Cyril Cohen, Enrico Tassi, Georges Gonthier, Laurence Rideau, Laurent Théry and Yves Bertot
Contact: Yves Bertot
Mathematical Components library
Functional Description: The Mathematical Components library is a set of Coq libraries that cover the mechanization of the proof of the Odd Order Theorem.
Release Functional Description: The library includes 16 more theory files, covering in particular field and Galois theory, advanced character theory, and a construction of algebraic numbers.
Participants: Alexey Solovyev, Andrea Asperti, Assia Mahboubi, Cyril Cohen, Enrico Tassi, François Garillot, Georges Gonthier, Ioana Pasca, Jeremy Avigad, Laurence Rideau, Laurent Théry, Russell O'Connor, Sidi Ould Biha, Stéphane Le Roux and Yves Bertot
Contact: Assia Mahboubi
Interval package for Coq
Keywords: Interval arithmetic - Coq
Functional Description: CoqInterval is a library for the proof assistant Coq.
It provides several tactics for proving theorems on enclosures of real-valued expressions. The proofs are performed by an interval kernel which relies on a computable formalization of floating-point arithmetic in Coq.
The Marelle team developed a formalization of rigorous polynomial approximation using Taylor models in Coq. In 2014, this library has been included in CoqInterval.
Participants: Assia Mahboubi, Érik Martin-Dorel, Guillaume Melquiond, Jean-Michel Muller, Laurence Rideau, Laurent Théry, Micaela Mayero, Mioara Joldes, Nicolas Brisebarre and Thomas Sibut-Pinote
Contact: Guillaume Melquiond
Publications: Proving bounds on real-valued functions with computations - Floating-point arithmetic in the Coq system - Proving Tight Bounds on Univariate Expressions with Elementary Functions in Coq - Formally Verified Approximations of Definite Integrals - Formally Verified Approximations of Definite Integrals
This year has seen the end of the writing and the publication
of a book on computer-algebra algorithms .
The course at Master 2 level Algorithmes efficaces en calcul formel
is a course that Alin Bostan and Frédéric Chyzak
have set up progressively since 2005
together with
Marc Giusti (LIX), Bruno Salvy (today AriC),
as well as, initially,
Éric Schost (LIX at the time) and François Ollivier (LIX),
and, more recently, Grégoire Lecerf (LIX).
The course is very strongly focused to presenting
the design of algorithms guided by complexity analysis,
with the goal to lead the students to the understanding
of all algorithmic aspects that are necessary to the “creative telescoping”
used for symbolic computations of sums and integrals.
Their lecture notes had been circulating in and used
by the (French) computer-algebra community,
while they long had the goal of turning them into a book.
They could publish it in 2017 (686 pages),
after a big finalization effort in 2016 and 2017.
The first parts of the book present fast algorithms
for basic objects (integers, polynomials, series, matrices, linear recurrences),
insisting on general principles to design efficient algorithms.
The next parts of the work build on them to address
topics that have made recent progress:
factorization of polynomials, algorithms for polynomial systems,
definite summation and integration.
The work is online as a HAL collection
In , Alin Bostan and Frédéric Chyzak,
together with Mark van Hoeij (Florida State University),
Manuel Kauers (Johannes Kepler University), and Lucien Pech,
have studied nearest-neighbors walks on the two-dimensional
square lattice, that is, models of walks on
Multiple binomial sums form a large class of multi-indexed sequences, closed under partial summation, which contains most of the sequences obtained by multiple summation of products of binomial coefficients, as well as all the sequences with algebraic generating function. Alin Bostan and Pierre Lairez, together with Bruno Salvy (AriC), have studied in the representation of the generating functions of binomial sums by integrals of rational functions. The outcome is twofold. Firstly, we have shown that a univariate sequence is a multiple binomial sum if and only if its generating function is the diagonal of a rational function. Secondly, we have proposed algorithms that decide the equality of multiple binomial sums and that compute recurrence relations for them. In conjunction with geometric simplifications of the integral representations, this approach behaves well in practice. The process avoids the computation of certificates and the problem of the appearance of spurious singularities that afflicts discrete creative telescoping, both in theory and in practice.
The diagonal of a multivariate power series
Counting lattice paths obeying various geometric constraints is a classical topic in combinatorics and probability theory. Many recent works deal with the enumeration of 2-dimensional walks with prescribed steps confined to the positive quadrant. A notoriously difficult case concerns the so-called Gessel walks: they are planar walks confined to the positive quarter plane, which move by unit steps in any of the West, North-East, East, and South-West directions. In 2001, Ira Gessel conjectured a closed-form expression for the number of such walks of a given length starting and ending at the origin. In 2008, Kauers, Koutschan and Zeilberger gave a computer-aided proof of this conjecture. The same year, Bostan and Kauers showed, using again computer algebra tools, that the trivariate generating function of Gessel walks is algebraic. This year, Alin Bostan, together with Irina Kurkova (Univ. Paris 6) and Kilian Raschel (CNRS and Univ. Tours), proposed in the first “human proofs” of these results. They are derived from a new expression for the generating function of Gessel walks in terms of special functions.
In , we have provided explicit formulae for the
coefficients of the order-
For matrices with displacement structure, basic operations like
multiplication, inversion, and linear-system solving can all be expressed in
terms of a single task:
evaluating the product
How many operations do we need on the average to compute an approximate root of
a random Gaussian polynomial system? Beyond Smale's 17th problem that asked
whether a polynomial bound is possible,
Pierre Lairez has proved in
a quasi-optimal bound
In , Pierre Lairez, jointly with Peter Bürgisser (TU Berlin) and Felipe Cucker (City University of Hong Kong), has described and analyzed an algorithm for computing the homology (Betti numbers and torsion coefficients) of basic semialgebraic sets. The algorithm works in weak exponential time, that is, out of a set of exponentially small measure in the space of data, the cost of the algorithm is exponential in the size of the data. All algorithms previously proposed for this problem have a complexity that is doubly exponential (and this is so for almost all data).
Assia Mahboubi and Thomas Sibut-Pinote, in collaboration with Guillaume Melquiond (Toccata), have pursued their work on the certified computation of intervals approximating the values of definite integrals involving elementary mathematical functions. This library provides an automated tool that builds a formal proof of the correctness of the output, that is, a formal proof that the interval contains the mathematical values and a formal proof of the integrability of the input function on the input interval. This tool has been extended this year, and it can now deal with improper integrals, that is, integrals whose bounds are infinite or singularities of the integrand. The methodology, the implementation and benchmarks have been described in .
Assia Mahboubi and Thomas Sibut-Pinote have completed a formal proof
of the irrationality of the constant
FastRelax (ANR-14-CE25-0018).
Goal: Develop computer-aided proofs of numerical values, with
certified and reasonably tight error bounds, without sacrificing
efficiency.
Leader: B. Salvy (Inria, ENS Lyon).
Participants: Assia Mahboubi, Th. Sibut-Pinote.
Website: http://
Marni Mishna (Simon Fraser University) visited the team for one week in January.
Emre Sertöz (Max Planck Institute Leipzig) visited the team for one week in November. He worked with Pierre Lairez on applications to algebraic geometry of two tools developped at Specfun: the computations of periods (Lairez's PhD) and numerical analytic continuation (Mezzarobba's PhD, 2011).
Karen Yeats (Simon Fraser University) visited the team for a few days in June. She continued a work on bijective combinatorics of words with Frédéric Chyzak. A text is now under writing.
Pascal Fong did a Master internship from March to August. Under the supervision of Pierre Lairez and Mohab Safey El Din (UPMC), he studied the numerical computation of the length of plane algebraic curves.
Rémy Garnier did a Master internship from March to July. Under the supervision of Alin Bostan and Frédéric Chyzak, he studied existing algorithms to solve linear differential systems for their rational-function solutions.
Meissa M'baye did a Master internship from February to June. Under the remote supervision of Assia Mahboubi, he studied the principles of proof assistants and surveyed formalization methodologies for elementary number theory.
Frédéric Chyzak and Alin Bostan have been invited by the Erwin Schrödinger
Institute (Vienna, Austria) for two weeks, to participate to the thematic
program “Algorithmic and Enumerative Combinatorics”
http://
Pierre Lairez visited Felipe Cucker (City University of Hong Kong) for two weeks. The outcome is a strengthened collaboration on the study of the complexity of numerical algorithms. A publication is in preparation: the second part of .
Georges Gonthier was invited at the Newton Institue, for six weeks, as co-organiser and participant to the Big Proof thematic program.
Assia Mahboubi visited Sander Dahmen (VU Amsterdam, The Netherlands) for three days. She has started a collaboration with his team, to obtain formal guarantees of computations for number theory.
Assia Mahboubi has been invited by the Newton Institute (Cambridge, UK) for one month. She participated to the Big Proof thematic program.
Alin Bostan is part of the Scientific advisory board of the conference series Effective Methods in Algebraic Geometry (MEGA).
Frédéric Chyzak is member of the steering committee of the Journées Nationales de Calcul Formel (JNCF), the annual meeting of the French computer algebra community.
Frédéric Chyzak is elected member of the steering committee of the International Symposium on Symbolic and Algebraic Computation (ISSAC, 3-year term, 2016–2018).
Assia Mahboubi has served on the scientific committee of the Journées Scientifiques Inria and of the EUTypes summer school.
Georges Gonthier was co-chair or the organising committee of the Big Proof thematic program held at the Newton Institute (Cambridge, UK) in June-August 2017.
Assia Mahboubi has organized the TTT workshop, satellite of the POPL'17 conference.
The team organizes a regular seminar
Assia Mahboubi has served as member of the conference program committees of the international conferences CPP 17, CADE 17, ITP 17, and on the program committee of the international workshops TYPES 17, TyDe 17, HoTT/UF 17.
Alin Bostan has served as reviewer for the selection of the international conferences ISSAC 2017 and MEGA 2017.
Frédéric Chyzak has served as reviewer for the selection of the international conference ISSAC 2017.
Assia Mahboubi has served as external reviewer for the international conference LICS 17.
Georges Gonthier is on the editorial board of the Journal of Formalized Reasoning.
Alin Bostan has served as a reviewer for the journals: Journal of Symbolic Computation; Linear Algebra and its Applications; Journal of Algebra and its Applications; Journal of Complexity; Advances in Applied Mathematics; Journal of Combinatorial Theory, Series A.
Assia Mahboubi has served as reviewer for the journals: Annals of Mathematics and Artificial Intelligence; Journal of Automated Reasoning.
Alin Bostan has been invited to give a series of three lectures
at the JNCF – Journées Nationales de Calcul Formel
(CIRM, Luminy, France), January 16–18, 2017,
http://
Alin Bostan has been invited to give a talk
at the workshop EDATE – Equations différentielles : aspects théoriques et effectifs, Grenoble, March 13–15, 2017,
http://
Alin Bostan has been invited to give a talk
at the workshop ANT – Automata in Number Theory,
île de Porquerolles, May 30–-June 2, 2017,
http://
Alin Bostan has been invited to give a talk
at the workshop Lattice walks at the Interface of Algebra, Analysis and Combinatorics,
BIRS, Banff, Canada, September 17–-22, 2017,
http://
Alin Bostan has been invited to give a series of introductory lectures
at the Workshop on Computer Algebra in Combinatorics, Erwin
Schrödinger Institut (ESI), Vienna, Austria, November 13–17, 2017,
http://
Frédéric Chyzak has been invited to give a talk
at the Second International Conference “Computer Algebra in Moscow”,
Plekhanov Russian University of Economics, Moscow, Russia,
October 30 to November 30, 2017,
http://
Frédéric Chyzak has been invited to give a talk
at the Workshop on Computer Algebra in Combinatorics, Erwin
Schrödinger Institut (ESI), Vienna, Austria, November 13–17, 2017,
http://
Georges Gonthier gave an invited talk at the Special Session on Computer-Aided Proofs of the Association for Symbolic Logic 2017 North American Meeting, Boise, Idaho, USA, March 20–23, 2017.
Georges Gonthier gave an invited talk at the Second Conference on Artificial Intelligence and Theorem Proving (AITP'17), Obergürgl, Austria, March 26–30, 2017.
Georges Gonthier gave a talk at the ERCIM Workshop on Blockchains, Paris, May 23, 2017.
Georges Gonthier gave an invited talk at the Workshop Computer-aided Mathematical Proof, part of the Big Proof Program, Isaac Newton Institute, Cambridge, U.K.
Pierre Lairez gave an invited talk at Effective Methods in Algebraic Geometry (MEGGA 2017), Nice.
Pierre Lairez gave an invited talk at the Conference on Foundations of Computational Mathematics (FoCM 2017), Barcelona, Spain.
Assia Mahboubi has been invited to give a talk at the General Mathematics Colloquium of the VU Amsterdam, The Netherlands.
Assia Mahboubi has been invited to give a talk at the Workshop Computer-aided Mathematical Proof, part of the Big Proof Program, Isaac Newton Institute, Cambridge, U.K.
Assia Mahboubi leads the working group Type theory based tools inside the EUTYPES COST project. She is also a member of the management committee for France for this project and a member of its core management group.
Georges Gonthier serves on the Conseil de l'École Doctorale de Mathématiques Hadamard.
Assia Mahboubi has been a member of the Commission Scientifique of Inria Saclay — Île-de-France, until September 2017.
Alin Bostan has served as a jury member of the French Agrégation de Mathématiques – épreuve de modélisation, option C.
Licence:
Thomas Sibut-Pinote, Les bases de la programmation et de l'algorithmique, 32h, L3, École polytechnique, France.
Thomas Sibut-Pinote, Les principes des langages de programmation, 32h, L3, École polytechnique, France.
Master:
Frédéric Chyzak, Algorithmes efficaces en calcul formel, 18h, M2, MPRI, France.
Alin Bostan, Algorithmes efficaces en calcul formel, 40.5h, M2, MPRI, France.
Pierre Lairez, Algorithmique avancée, 18h, M1, École polytechnique, France.
Assia Mahboubi, Algorithmes d'élimination des quantificateurs, 3h, M2, Université Rennes 1, France.
HdR : Alin Bostan, Computer algebra for lattice path combinatorics , Université Paris 13, December 15, 2017.
PhD : Thomas Sibut-Pinote, Investigations en Mathématiques Assistées par Ordinateur: Expérimentation, Calcul et Certification, Université Paris-Saclay, December 4, 2017.
Frédéric Chyzak has served as an examiner in the PhD jury of Cyril Hugounenq Volcans et calcul d'isogénies, Université de Versailles – Saint-Quentin-en-Yvelines, September 25, 2017.
Frédéric Chyzak has been a member of the hiring jury at Inria (Concours CR 2017).
Georges Gonthier served on the Habilitation à diriger des Recherches of Paul-André Melliès Une étude micrologique de la négation, Université Paris Diderot, November 20, 2017.
Assia Mahboubi has been a member of the hiring jury at Inria (Concours CR 2017).
Assia Mahboubi has served as an examiner in the PhD jury of Evmorfia-Iro Bartzia Une formalisation des courbes elliptiques pour la cryptographie, Université Paris-Saclay, February 15, 2017.
Assia Mahboubi has served as an examiner in the PhD jury of Étienne Miquey Réalisabilité classique et effets de bord, Université Paris Diderot, November 17, 2017.
Assia Mahboubi has been a member of the hiring jury of a Maître de conférence position at Université Paris Diderot.
Assia Mahboubi has written an article for the MathExpress
journal, at the occasion of the salon Culture & Jeux Mathématiques.
See the Maths Language express volume at
http://