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A173442 Number of divisors d of number n such that sigma(d) does not divide n. 2
0, 1, 1, 2, 1, 2, 1, 3, 2, 3, 1, 2, 1, 3, 3, 4, 1, 4, 1, 5, 3, 3, 1, 4, 2, 3, 3, 4, 1, 5, 1, 5, 3, 3, 3, 5, 1, 3, 3, 7, 1, 6, 1, 5, 5, 3, 1, 6, 2, 5, 3, 5, 1, 6, 3, 4, 3, 3, 1, 7, 1, 3, 5, 6, 3, 6, 1, 5, 3, 7, 1, 8, 1, 3, 5, 5, 3, 6, 1, 9, 4, 3, 1, 6, 3, 3, 3, 7, 1, 8, 3, 5, 3, 3, 3, 8, 1, 5, 5, 8 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,4
COMMENTS
Sigma(n) = A000203(n). a(n) = A000005(n) - A173441(n).
a(n) >= 1 for n >= 2, with equality if and only if n is prime. - Robert Israel, Oct 10 2017
LINKS
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
Sequence in context: A261350 A259177 A304036 * A112309 A160006 A060682
KEYWORD
nonn
AUTHOR
Jaroslav Krizek, Feb 18 2010
EXTENSIONS
More terms from Robert Israel, Oct 10 2017
STATUS
approved

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Last modified July 19 20:58 EDT 2024. Contains 374436 sequences. (Running on oeis4.)