On $n$-Dimensional Sequences. I

TL;DR

This paper investigates the algebraic structure of $n$-dimensional sequences over a commutative ring, providing explicit formulas for their generating functions and characteristic polynomial ideals, and introduces algorithms for computing these ideals.

Contribution

It offers explicit partitioned sum expressions for the generating functions and characteristic ideals of $n$-dimensional sequences, and develops algorithms for their computation over factorial domains.

Findings

Explicit $2^n$-fold sum formulas for generating functions.

Characterization of the ideal Ann$(s)$ for eventually rectilinear sequences.

Polynomial-time algorithms for computing characteristic ideals over fields.

Abstract

Let $R$ be a commutative ring and let $n \geq 1.$ We study $\Gamma(s)$, the generating function and Ann$(s)$, the ideal of characteristic polynomials of $s$, an $n$--dimensional sequence over $R$. We express $f(X_1,\ldots,X_n) \cdot \Gamma(s)(X_1^{-1},\ldots ,X_n^{-1})$ as a partitioned sum. That is, we give (i) a $2^n$--fold ``border'' partition (ii) an explicit expression for the product as a $2^n$--fold sum; the support of each summand is contained in precisely one member of the partition. A key summand is $\beta_0(f,s)$, the ``border polynomial'' of $f$ and $s$, which is divisible by $X_1\cdots X_n$. We say that $s$ is {\em eventually rectilinear} if the elimination ideals Ann$(s)\cap R[X_i]$ contain an $f_i(X_i)$ for $1 \leq i \leq n$. In this case, we show that $\mbox{Ann}(s)$ is the ideal quotient $(\sum_{i=1}^n(f_i)\ :\ \beta_0(f,s)/(X_1\cdots X_n)).$ When $R$ and $R[[X_1,X_2, \ldots ,X_n]]$ are factorial domains (e.g. $R$ a principal ideal domain or ${\Bbb F}[X_1,\ldots,X_n]$), we compute {\em the monic generator} $\gamma _i$ of $\mbox{Ann}(s) \cap R[X_i]$ from known $f_i \in \mbox{Ann}(s) \cap R[X_i]$ or from a finite number of $1$--dimensional linear recurring sequences over $R$. Over a field ${\Bbb F}$ this gives an $O(\prod_{i=1}^n \delta \gamma _i^3)$ algorithm to compute an ${\Bbb F}$--basis for $\mbox{Ann}(s)$.