OFFSET
1,3
COMMENTS
a(n) = Card[Intersection[D[A000203(n)], D[A001157(n)]]]. - This is the formula given by the original author. D[x] here means the set of divisors of x. - Antti Karttunen, Nov 23 2017
LINKS
Antti Karttunen, Table of n, a(n) for n = 1..16384
FORMULA
EXAMPLE
n=10: sigma[1,10]=18, sigma[1,10]=130 Intersection[{1,2,3,6,9,18},{1,2,5,10,13,26,65,130}]={1,2}, so a(10)=2.
MATHEMATICA
g1[x_] := Divisors[DivisorSigma[1, x]] g2[x_] := Divisors[DivisorSigma[2, x]] ncd[x_] := Length[Intersection[g1[x], g2[x]]] Table[ncd[w], {w, 1, 128}]
(* Second program: *)
Table[Length@ Apply[Intersection, Divisors@ Array[DivisorSigma[#, n] &, 2]], {n, 105}] (* Michael De Vlieger, Nov 23 2017 *)
PROG
(PARI) A073808(n) = numdiv(gcd(sigma(n), sigma(n, 2))); \\ Antti Karttunen, Nov 23 2017
CROSSREFS
KEYWORD
nonn
AUTHOR
Labos Elemer, Aug 13 2002
STATUS
approved