a(n) = [x^n] Product_{k>=1} 1/(1 - x^(k*(k+1)/2))^n.

Terms

a(0) =1a(1) =1a(2) =3a(3) =13a(4) =51a(5) =201a(6) =825a(7) =3431a(8) =14355a(9) =60493a(10) =256463a(11) =1092268a(12) =4669665a(13) =20029036a(14) =86148373a(15) =371434173a(16) =1604845715a(17) =6946936628a(18) =30121158813a(19) =130795358333a(20) =568709929191