OFFSET
1,5
COMMENTS
Also number of divisors of 5*n-1 of form 5*k+4.
LINKS
Seiichi Manyama, Table of n, a(n) for n = 1..10000
FORMULA
MATHEMATICA
a[n_] := DivisorSum[5*n-1, 1 &, Mod[#, 5] == 1 &]; Array[a, 100] (* Amiram Eldar, Aug 23 2023 *)
PROG
(PARI) a(n) = sumdiv(5*n-1, d, d%5==1);
(PARI) a(n) = sumdiv(5*n-1, d, d%5==4);
(PARI) my(N=100, x='x+O('x^N)); Vec(sum(k=1, N, x^k/(1-x^(5*k-1))))
(PARI) my(N=100, x='x+O('x^N)); Vec(sum(k=1, N, x^(4*k-3)/(1-x^(5*k-4))))
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Seiichi Manyama, Dec 22 2022
STATUS
approved