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%I #13 Mar 13 2020 20:36:20
%S 0,1,1,2,2,1,1,1,3,2,2,1,2,1,2,2,3,2,2,2,4,2,2,2,2,4,3,3,4,3,2,1,5,2,
%T 4,2,4,4,2,3,5,2,3,3,3,4,5,5,4,2,4,3,6,3,2,5,6,2,3,2,5,2,2,4,5,3,3,2,
%U 3,1,4,4,5,3,5,4,9,3,3,3,5,4,5,4,3,4
%N Number of prime factors of form cn+1 for numbers 3^n-1
%H Seppo Mustonen, <a href="/A182591/b182591.txt">Table of n, a(n) for n = 2..170</a>
%H S. Mustonen, <a href="http://www.survo.fi/papers/MustonenPrimes.pdf">On prime factors of numbers m^n+-1</a>
%H Seppo Mustonen, <a href="/A182590/a182590.pdf">On prime factors of numbers m^n+-1</a> [Local copy]
%e For n=6, 3^n-1=728 has two prime factors of the form cn+1, namely 7=n+1 and 13=2n+1. Thus a(6)=2.
%t m = 3; n = 2; nmax = 170;
%t While[n <= nmax, {l = FactorInteger[m^n - 1]; s = 0;
%t For[i = 1, i <= Length[l],
%t i++, {p = l[[i, 1]];
%t If[IntegerQ[(p - 1)/n] == True, s = s + l[[i, 2]]];}];
%t a[n] = s;} n++;];
%t Table[a[n], {n, 2, nmax}]
%K nonn
%O 2,4
%A _Seppo Mustonen_, Nov 22 2010
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