On numbers n dividing the nth term of a linear recurrence Juan Jose´ Alba Gonzalez´ Instituto de Matem´aticas, Universidad Nacional Autonoma de M´exico, C.P. 58089, Morelia, Michoac´an, M´exico
[email protected] Florian Luca Instituto de Matem´aticas, Universidad Nacional Autonoma de M´exico, C.P. 58089, Morelia, Michoac´an, M´exico
[email protected] Carl Pomerance Mathematics Department, Dartmouth College, Hanover, NH 03755, USA
[email protected] Igor E. Shparlinski Dept. of Computing, Macquarie University Sydney, NSW 2109, Australia
[email protected] Abstract Here, we give upper and lower bounds on the count of positive integers n x dividing the nth term of a nondegenerate linearly ≤ recurrent sequence with simple roots. 1 1 Introduction Let un n 0 be a linear recurrence sequence of integers satisfying a homoge- ≥ neous{ linear} recurrence relation un+k = a1un+k 1 + + ak 1un+1 + akun, for n =0, 1,..., (1) − ··· − where a ,...,a are integers with a = 0. 1 k k In this paper, we study the set of indices n which divide the corresponding term un; that is, the set: := n 1 : n u . Nu { ≥ | n} But first, some background on linear recurrence sequences. To the recurrence (1), we associate its characteristic polynomial m k k 1 σi fu(X) := X a1X − ak 1X ak = (X αi) Z[X], − −···− − − − ∈ i=1 where α ,...,α C are the distinct roots of f (X) with multiplicities 1 m ∈ u σ1,...,σm, respectively. It is then well-known that the general term of the recurrence can be expressed as m n un = Ai(n)αi , for n =0, 1,..., (2) i=1 where A (X) are polynomials of degrees at most σ 1 for i =1,...,m, with i i − coefficients in K := Q[α1,...,αm].