OFFSET
1,1
FORMULA
a(1)=7 because set of prime divisors of 7^2-1 is the same as for 5^2-1. The set is {2,3}.
MATHEMATICA
aa = {}; bb = {}; Do[k = n^2 - 1; c = FactorInteger[k]; b = {}; Do[AppendTo[b, c[[m]][[1]]], {m, 1, Length[c]}]; If[Position[aa, b] != {}, AppendTo[bb, n], AppendTo[aa, b]], {n, 2, 10000}]; bb (*Artur Jasinski*)
PROG
(PARI) isok(n) = {pfs = factor(n^2-1)[, 1]; for (k = 2, n-1, if (factor(k^2-1)[, 1] == pfs, return (1)); ); return (0); } \\ Michel Marcus, Nov 04 2013
CROSSREFS
KEYWORD
nonn
AUTHOR
Artur Jasinski, Oct 11 2010
EXTENSIONS
Edited by N. J. A. Sloane, Oct 14 2010
STATUS
approved