Expansion of Product_{k>=1} 1 / (1 - 3*x^k)^2.

Terms

a(0) =1a(1) =6a(2) =33a(3) =150a(4) =636a(5) =2508a(6) =9501a(7) =34674a(8) =123369a(9) =429396a(10) =1469733a(11) =4959600a(12) =16545597a(13) =54662046a(14) =179124837a(15) =582893052a(16) =1885479918a(17) =6067245570a(18) =19435083054a(19) =62006825166a(20) =197128631562a(21) =624716063502