Section: New Results

Algebraic diagonals and walks

The diagonal of a multivariate power series $F$ is the univariate power series $\mathrm{𝖣𝗂𝖺𝗀}F$ generated by the diagonal terms of $F$. Diagonals form an important class of power series; they occur frequently in number theory, theoretical physics and enumerative combinatorics. In  [35], Alin Bostan and Louis Dumont, together with Bruno Salvy (Inria and ENS Lyon), have studied algorithmic questions related to diagonals in the case where $F$ is the Taylor expansion of a bivariate rational function. It is classical that in this case $\mathrm{𝖣𝗂𝖺𝗀}F$ is an algebraic function. We propose an algorithm that computes an annihilating polynomial for $\mathrm{𝖣𝗂𝖺𝗀}F$. We give a precise bound on the size of this polynomial and show that generically, this polynomial is the minimal polynomial and that its size reaches the bound. The algorithm runs in time quasi-linear in this bound, which grows exponentially with the degree of the input rational function. We then address the related problem of enumerating directed lattice walks. The insight given by our study leads to a new method for expanding the generating power series of bridges, excursions and meanders. We show that their first $N$ terms can be computed in quasi-linear complexity in $N$, without first computing a very large polynomial equation. An extended version of this work is presented in [3].