Expansion of Sum_{1<=i<=j<=k<=l} q^(i+j+k+l)/( (1-q^i)*(1-q^j)*(1-q^k)*(1-q^l) )^2.

Terms

a(0) =1a(1) =9a(2) =44a(3) =156a(4) =450a(5) =1121a(6) =2508a(7) =5139a(8) =9867a(9) =17831a(10) =30888a(11) =51117a(12) =82212a(13) =127458a(14) =193809a(15) =285702a(16) =415701a(17) =588797a(18) =826803a(19) =1133352a(20) =1545986a(21) =2062203a(22) =2744811a(23) =3579155a(24) =4665375a(25) =5964543a(26) =7638983a(27) =9600392a(28) =12104136a(29) =14992496