# New Congruences and Finite Di¯¬â‚¬erence Equations for...

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New Congruences and Finite Difference Equations for Generalized Factorial Functions

Maxie D. Schmidt University of Washington

Department of Mathematics

Padelford Hall Seattle, WA 98195

USA maxieds@gmail.com

Abstract

We use the rationality of the generalized hth convergent functions, Convh(α,R; z), to the infinite J-fraction expansions enumerating the generalized factorial product se- quences, pn(α,R) = R(R+α) · · · (R+ (n− 1)α), defined in the references to construct new congruences and h-order finite difference equations for generalized factorial func- tions modulo hαt for any primes or odd integers h ≥ 2 and integers 0 ≤ t ≤ h. Special cases of the results we consider within the article include applications to new congru- ences and exact formulas for the α-factorial functions, n!(α). Applications of the new results we consider within the article include new finite sums for the α-factorial func- tions, restatements of classical necessary and sufficient conditions of the primality of special integer subsequences and tuples, and new finite sums for the single and double factorial functions modulo integers h ≥ 2.

1 Notation and other conventions in the article

1.1 Notation and special sequences

Most of the conventions in the article are consistent with the notation employed within the Concrete Mathematics reference, and the conventions defined in the introduction to the first articles [11, 12]. These conventions include the following particular notational variants:

◮ Extraction of formal power series coefficients. The special notation for formal power series coefficient extraction, [zn]

(∑

k fkz k ) : 7→ fn;

◮ Iverson’s convention. The more compact usage of Iverson’s convention, [i = j]δ ≡ δi,j, in place of Kronecker’s delta function where [n = k = 0]δ ≡ δn,0δk,0;

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http://arxiv.org/abs/1701.04741v1 mailto:maxieds@gmail.com

◮ Bracket notation for the Stirling number triangles. We use the alternate bracket notation for the Stirling number triangles,

[ n k

] = (−1)n−ks(n, k) and

{ n k

} =

S(n, k);

◮ Harmonic number sequences. Use of the notation for the first-order harmonic numbers, Hn or H

(1) n , which defines the sequence

Hn := 1 + 1

2 +

1

3 + · · ·+

1

n ,

and the notation for the partial sums for the more general cases of the r-order harmonic numbers, H

(r) n , defined as

H (r) n := 1 + 2

−r + 3−r + · · ·+ n−r,

when r, n ≥ 1 are integer-valued and where H (r) n ≡ 0 for all n ≤ 0;

◮ Rising and falling factorial functions. We use the convention of denoting the falling factorial function by xn = x!/(x − n)!, the rising factorial function as xn = Γ(x + n)/Γ(x), or equivalently by the Pochhammer symbol, (x)n = x(x + 1)(x + 2) · · · (x+ n− 1);

◮ Shorthand notation in integer congruences and modular arithmetic. Within the article the notation g1(n) ≡ g2(n) (mod N1, N2, . . . , Nk) is understood to mean that the congruence, g1(n) ≡ g2(n) (mod Nj), holds modulo any of the bases, Nj , for 1 ≤ j ≤ k.

The standard set notation for Z, Q, and R denote the sets of integers, rational numbers, and real numbers, respectively, where the set of natural numbers, N, is defined by N := {0, 1, 2, . . .} = Z+

⋃ {0}. Other more standard notation for the special functions cited within

the article is consistent with the definitions employed in the NIST Handbook of Mathematical Functions (2010).

1.2 Mathematica summary notebook document and computational reference information

The article is prepared with a more extensive set of computational data and software routines released as open source software to accompany the examples and numerous other applica- tions suggested as topics for future research and investigation within the article. It is highly encouraged, and expected, that the interested reader obtain a copy of the summary notebook reference and computational documentation prepared in this format to assist with compu- tations in a multitude of special case examples cited as particular applications of the new results.

The prepared summary notebook file, multifact-cfracs-summary.nb , attached to the submission of this manuscript contains the working Mathematica code to verify the formulas,

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propositions, and other identities cited within the article [13]. Given the length of this and the first article, the Mathematica summary notebook included with this submission is intended to help the reader with verifying and modifying the examples presented as applications of the new results cited below. The summary notebook also contains numerical data corresponding to computations of multiple examples and congruences specifically referenced in several places by the applications given in the next sections of the article.

2 Introduction

2.1 Motivation

In this article, we extend the results from the reference [11] providing infinite J-fraction expansions for the typically divergent ordinary generating functions (OGFs) of generalized factorial product sequences of the form

pn (α,R) := R(R + α)(R + 2α)× · · · × (R + (n− 1)α) [n ≥ 1]δ + [n = 0]δ , (1)

when R depends linearly on n. Notable special cases of (1) that we are particularly interested in enumerating through the convergents to these J-fraction expansions include the multiple, or α-factorial functions, n!(α), defined for α ∈ Z

+ as

n!(α) =

n · (n− α)!(α), if n > 0;

1, if −α < n ≤ 0;

0, otherwise,

(2)

and the generalized factorial functions of the form pn(α, βn + γ) for α, β, γ ∈ Z, α 6= 0, and β, γ not both zero. The second class of special case products are related to the Gould polynomials, Gn(x; a, b) =

x x−an

· ( x−an

b

)n , through the following identity ([12, §3.4.2],[10,

§4.1.4]):

pn (α, βn+ γ) = (−α)n+1

γ − α− β ×Gn+1 (γ − α− β;−β,−α) . (3)

The α-factorial functions, (αn− d)!(α) for α ∈ Z + and some 0 ≤ d < α, form special cases

of (3) where, equivalently, (α, β, γ) ≡ (−α, α,−d) and (α, β, γ) ≡ (α, 0, α− d) [11, §6]. The α-factorial functions are expanded by the triangles of Stirling numbers of the first kind,

[ n k

] ,

and the α-factorial coefficients, [ n k

]

α , respectively, in the following forms [5, 12]:

n!(α) = n∑

m=0

[ ⌈n/α⌉

m

]

(−α)⌈ n α ⌉−mnm, ∀n ≥ 1, α ∈ Z+ (4)

= n∑

m=0

[ ⌊n−1+α

α ⌋+ 1

m+ 1

]

α

(−1)⌊ n−1+α

α ⌋−m(n+ 1)m, ∀n ≥ 1, α ∈ Z+

3

(αn− d)!(α) = (α− d)× n∑

m=1

[ n

m

]

α

(−1)n−m(αn+ 1− d)m−1

=

n∑

m=0

[ n + 1

m+ 1

]

α

(−1)n−m(αn+ 1− d)m, ∀n ≥ 1, α ∈ Z+, 0 ≤ d < α.

A careful treatment of the polynomial expansions of these generalized α-factorial functions through the coefficient triangles in (4) is given in the reference [12].

2.2 Summary of the J-fraction results

For all h ≥ 2, we can generate the generalized factorial product sequences, pn(α,R), through the strictly rational generating functions provided by the hth convergent functions, denoted by Convh (α,R; z), to the infinite continued fraction series established by the reference [11]. In particular, we have series expansions of these convergent functions given by

Convh (α,R; z) := 1

1− R · z − αR · z2

1− (R + 2α) · z − 2α(R + α) · z2

1− (R + 4α) · z − 3α(R + 2α) · z2

· · ·

1− (R + 2(h− 1)α) · z

= FPh(α,R; z)

FQh(α,R; z) (5)

= h∑

n=0

pn(α,R)z n +

∞∑

n>h

[pn(α,R) (mod h)] z n,

where the convergent function numerator and denominator polynomial subsequences provid- ing the characteristic expansions of (5) are given in closed-form by

FQh(α,R; z) =

h∑

k=0

( h

k

)

(−1)k

( k−1∏

j=0

(R + (h− 1− j)α)

)

zk (6)

=

h∑

k=0

( h

k

)( R

α + h− k

)

k

(−αz)k

= (−αz)h · h!× L (R/α−1) h

( (αz)−1

) ,

when L (β) n (x) denotes an associated Laguerre polynomial, and where

FPh(α,R; z) = h−1∑

n=0

Ch,n(α,R)z n (7a)

4

=

h−1∑

n=0

( n∑

i=0

( h

i

)

(−1)ipi (−α,R + (h− 1)α) pn−i (α,R)

)

zn (7b)

=

h−1∑

n=0

( n∑

i=0

( h

i

)

(1− h− R/α)i (R/α)n−i

)

(αz)n. (7c)

The coefficients of the polynomial powers of z in the previous several expansions, Ch,n(α,R) := [zn] FPh(α,R; z) for 0 ≤ n < h, also have the fol

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