Counting walks with large steps in an orthant
Résumé
In the past fifteen years, the enumeration of lattice walks with steps taken
in a prescribed set S and confined to a given cone, especially the first
quadrant of the plane, has been intensely studied. As a result, the generating functions of
quadrant walks are now well-understood, provided the allowed steps are
small, that is $S \subset \{-1, 0,1\}^2$. In particular, having small
steps is crucial for the definition of a certain group of bi-rational
transformations of the plane. It has been proved that this group is finite if
and only if the corresponding generating function is D-finite (that is, it satisfies a linear
differential equation with polynomial coefficients). This group is also the
key to the uniform solution of 19 of the 23 small step models possessing a
finite group.
In contrast, almost nothing is known for walks with arbitrary steps. In this
paper, we extend the definition of the group, or rather of the associated
orbit, to this general case, and generalize the above uniform solution of
small step models. When this approach works, it invariably yields a D-finite
generating function. We apply it to many quadrant problems, including some infinite families.
After developing the general theory, we consider the $13\ 110$ two-dimensional
models with steps in $\{-2,-1,0,1\}^2$ having at least one $-2$ coordinate. We
prove that only 240 of them have a finite orbit, and solve 231 of them with
our method. The 9 remaining models are the counterparts of the 4 models of the
small step case that resist the uniform solution method (and which are known
to have an algebraic generating function). We conjecture D-finiteness for their generating
functions, but only two of them are likely to be algebraic. We also prove
non-D-finiteness for the $12\ 870$ models with an infinite orbit, except for
16 of them.
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