OFFSET
1,1
COMMENTS
A prime p is inefficient (see Amdeberhan - Moll) if it divides A000085(n) for some n < p.
LINKS
Robert Israel, Table of n, a(n) for n = 1..1428
Tewodros Amdeberhan and Victor H. Moll, Involutions and their progenies, preprint, 2014.
Tewodros Amdeberhan and Victor H. Moll, Involutions and their progenies, arXiv:1406.2356 [math.CO], 2014.
Tewodros Amdeberhan and Victor H. Moll, Involutions and their progenies, Journal of Combinatorics, 6(4) (2015), 483-508.
MAPLE
N:= 1000: # to get all terms <= N
I1:= proc(n) option remember; I1(n-1)+(n-1)*I1(n-2) end proc:
I1(0):= 1: I1(1):= 1:
Primes:= select(isprime, {2, seq(2*i+1, i=1..floor((N-1)/2))}):
PP:= convert(Primes, `*`):
A:= {}:
for n from 1 to N-1 do
g:= igcd(I1(n), PP):
A:= A union select(`>`, numtheory:-factorset(g), n);
od:
A; # Robert Israel, Jul 20 2014
MATHEMATICA
A85 = DifferenceRoot[Function[{y, n}, {(-n-1) y[n] - y[n+1] + y[n+2] == 0, y[1] == 1, y[2] == 2}]];
inefficientQ[p_] := AnyTrue[Range[p-1], Divisible[A85[#], p]&];
Reap[For[p = 2, p < 1000, p = NextPrime[p], If[inefficientQ[p], Print[p]; Sow[p]]]][[2, 1]] (* Jean-François Alcover, Jul 28 2020 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
N. J. A. Sloane, Jul 19 2014
STATUS
approved