A363898 - OEIS

%I #18 Jun 29 2023 13:22:00

%S 0,1,0,2,0,3,1,4,0,5,0,7,0,9,0,8,1,9,0,10,3,12,0,14,0,13,1,18,0,15,0,

%T 17,0,19,5,21,1,19,0,20,0,28,0,24,0,23,1,28,7,25,3,27,0,29,0,36,1,29,

%U 0,35,0,32,9,34,0,36,1,38,0,45,0,43,0,39,0,38,12,39,0,40,3,42,0,63,5,43,1

%N Expansion of Sum_{k>0} k * x^(2*k) / (1 - x^(5*k)).

%H Seiichi Manyama, <a href="/A363898/b363898.txt">Table of n, a(n) for n = 1..10000</a>

%F a(n) = Sum_{d|n, n/d==2 mod 5} d.

%F G.f.: Sum_{k>0} x^(5*k-3) / (1 - x^(5*k-3))^2.

%t a[n_] := DivisorSum[n, # &, Mod[n/#, 5] == 2 &]; Array[a, 100] (* _Amiram Eldar_, Jun 27 2023 *)

%o (PARI) a(n) = sumdiv(n, d, (n/d%5==2)*d);

%Y Cf. A363897, A363899, A363900.

%Y Cf. A001877, A284280.

%K nonn

%O 1,4

%A _Seiichi Manyama_, Jun 27 2023