OFFSET
1,8
FORMULA
a(n) = (1/5) * Sum_{d | 5*n-4, d==4 (mod 5)} (d+1).
G.f.: Sum_{k>0} x^(4*k) / (1 - x^(5*k-1))^2.
MATHEMATICA
a[n_] := DivisorSum[5*n - 4, # + 1 &, Mod[#, 5] == 4 &]/5; Array[a, 100] (* Amiram Eldar, Jul 12 2023 *)
PROG
(PARI) a(n) = sumdiv(5*n-4, d, (d%5==4)*(d+1))/5;
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Jul 05 2023
STATUS
approved