G.f.: Sum_{n=-oo..+oo} x^n * (1 - x^n)^n / (1 - x)^n.

Terms

a(0) =1a(1) =2a(2) =1a(3) =8a(4) =7a(5) =27a(6) =45a(7) =102a(8) =194a(9) =439a(10) =844a(11) =1775a(12) =3608a(13) =7342a(14) =14891a(15) =30283a(16) =61113a(17) =123625a(18) =249355a(19) =502430a(20) =1011305a(21) =2034028a(22) =4086860a(23) =8206874a(24) =16469851a(25) =33035697a(26) =66234208a(27) =132746099a(28) =265961186a(29) =532718115