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%I #15 Jul 26 2015 14:41:14
%S 2,5,14,20,39,52,74,102,134,158,208,259,284,361,409,488,538,634,678,
%T 838,857,1006,1038,1270,1264,1495,1500,1776,1761,2084,2062,2443,2300,
%U 2795,2680,3194,3076,3544,3403,4080,3804,4518,4282,5037,4673,5626,5127,6088
%N Total number of parts in all partitions of n into 2 sizes of parts (A002133).
%C Also column k=2 of A255768. - _Omar E. Pol_, Jul 26 2015
%H Alois P. Heinz, <a href="/A216669/b216669.txt">Table of n, a(n) for n = 3..1000</a>
%F a(n) = Sum_{k=2..n-1} k * A216665(n,k).
%t nn=40;ss=Sum[Sum[y^2 x^(i+j)/(1-y x^i)/(1-y x^j),{j,1,i-1}], {i,1,nn}]; CoefficientList[Series[D[ss,y]/.y->1,{x,0,nn}],x]
%Y Cf. A002133, A216665, A255768.
%K nonn
%O 3,1
%A _Geoffrey Critzer_, Sep 13 2012