OFFSET
1,4
COMMENTS
a(n) >= 1 for n >= 2, with equality if and only if n is prime. - Robert Israel, Oct 10 2017
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
EXAMPLE
For n = 12, a(12) = 2. We see that the divisors of 12 are 1, 2, 3, 4, 6, 12. The corresponding sigma(d) are 1, 3, 4, 7, 12, 28. The sigma(d) which do not divide n for 2 divisors d are 4 and 12.
MAPLE
f:= n -> nops(select(t -> n mod numtheory:-sigma(t) <> 0, numtheory:-divisors(n))):
map(f, [$1..100]); # Robert Israel, Oct 10 2017
MATHEMATICA
Table[Length[Select[Divisors[n], Not[Divisible[n, DivisorSigma[1, #]]], &]], {n, 100}] (* Alonso del Arte, Oct 10 2017 *)
PROG
(PARI) a(n) = sumdiv(n, d, (n % sigma(d)) != 0); \\ Michel Marcus, Oct 11 2017
CROSSREFS
KEYWORD
nonn
AUTHOR
Jaroslav Krizek, Feb 18 2010
EXTENSIONS
More terms from Robert Israel, Oct 10 2017
STATUS
approved