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Expansion of Sum_{n>=0} x^(n*(n+1)/2) * Product_{k=0..n} (1 + x^(k+1)).
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%I #12 Jan 03 2021 14:08:05

%S 1,2,1,2,2,1,3,2,2,3,3,3,3,4,3,5,5,4,5,5,6,7,7,6,8,8,8,9,10,10,11,12,

%T 11,13,13,14,16,16,16,18,18,19,21,22,22,24,26,25,28,29,30,32,33,34,37,

%U 39,40,42,44,45,48,50,51,55,57,59,63,65,66,71,73,76,80,83,85,90,94,96,102,105,109,115,118,122,128,133,137,143,149,153,161,167,172,179,186,191,200

%N Expansion of Sum_{n>=0} x^(n*(n+1)/2) * Product_{k=0..n} (1 + x^(k+1)).

%C Sum_{n>=0} x^(n*(n+1)/2) / Product_{k=0..n} (1 + x^(k+1)) = 1.

%C What does this sequence count?

%o (PARI) N=66; x='x + O('x^N); s =sum(n=0,N, x^(n*(n+1)/2) * prod(k=0,n, 1+x^(k+1) ) ); v=Vec(s)

%K nonn

%O 0,2

%A _Joerg Arndt_, Aug 14 2011