A336387 Number of prime divisors of n that do not divide sigma(n); a(1) = 0.
0, 1, 1, 1, 1, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 1, 0, 1, 1, 1, 0, 1, 1, 1, 1, 1, 1, 2, 2, 1, 1, 2, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 1, 1, 0, 2, 1, 2, 1, 1, 1, 1, 1, 2, 1, 2, 1, 1, 1, 1, 2, 1, 1, 1, 1, 2, 1, 2, 1, 1, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 1, 2, 1, 1, 0, 1, 2, 1, 2, 1, 1, 1, 1, 2
Offset: 1
Keywords
Links
Programs
-
Mathematica
Table[Length[Select[FactorInteger[n][[All,1]],Mod[DivisorSigma[ 1,n],#]!= 0&]],{n,110}] (* Harvey P. Dale, Jan 15 2022 *) -
PARI
A336387(n) = if(1==n,0,my(s=sigma(n)); #select(p -> (s%p), factor(n)[, 1]));
Formula
a(n) = Sum_{p over distinct primes dividing n} [sigma(n) != 0 mod p].