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

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =2a(5) =4a(6) =8a(7) =20a(8) =47a(9) =104a(10) =246a(11) =607a(12) =1496a(13) =3751a(14) =9579a(15) =24720a(16) =64327a(17) =168932a(18) =446830a(19) =1188030a(20) =3177198a(21) =8541152a(22) =23063100a(23) =62550085a(24) =170337684a(25) =465564180a(26) =1276779917a(27) =3512617527a(28) =9692054125a(29) =26815357935