G.f.: Sum_{n>=0} x^n * (1 + x^n)^n / (1 - x + x^(n+1))^(n+1).

Terms

a(0) =1a(1) =1a(2) =4a(3) =7a(4) =12a(5) =23a(6) =55a(7) =116a(8) =228a(9) =443a(10) =885a(11) =1812a(12) =3743a(13) =7635a(14) =15391a(15) =30890a(16) =62033a(17) =124795a(18) =251329a(19) =506102a(20) =1018194a(21) =2045892a(22) =4106530a(23) =8236843a(24) =16514499a(25) =33102119a(26) =66336493a(27) =132908840a(28) =266228896a(29) =533159832