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

Terms

a(0) =1a(1) =1a(2) =2a(3) =2a(4) =3a(5) =5a(6) =7a(7) =14a(8) =20a(9) =27a(10) =49a(11) =87a(12) =128a(13) =189a(14) =345a(15) =608a(16) =896a(17) =1306a(18) =2216a(19) =3921a(20) =6460a(21) =10068a(22) =15605a(23) =25039a(24) =42584a(25) =73806a(26) =120926a(27) =184288a(28) =281330a(29) =466014