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Sum of divisors of 3*n-1 of form 3*k+2.
5

%I #18 Jun 27 2023 09:17:03

%S 2,5,10,11,16,17,27,23,28,29,42,40,40,41,57,47,57,53,80,59,64,70,87,

%T 71,76,88,115,83,88,89,117,100,114,101,140,107,128,113,147,136,124,

%U 130,170,131,136,137,216,154,148,149,200,160,160,184,207,167,194,173,241,179,224,190,237

%N Sum of divisors of 3*n-1 of form 3*k+2.

%H Seiichi Manyama, <a href="/A363890/b363890.txt">Table of n, a(n) for n = 1..10000</a>

%F a(n) = A078182(3*n-1).

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

%t a[n_] := DivisorSum[3*n - 1, # &, Mod[#, 3] == 2 &]; Array[a, 100] (* _Amiram Eldar_, Jun 26 2023 *)

%o (PARI) a(n) = sumdiv(3*n-1, d, (d%3==2)*d);

%Y Cf. A363514, A363889, A363891.

%Y Cf. A078182, A359211.

%K nonn

%O 1,1

%A _Seiichi Manyama_, Jun 26 2023