G.f.: Sum_{n>=0} (2 + x^n)^n * x^n / (1-x)^(n+1).

Terms

a(0) =1a(1) =3a(2) =10a(3) =29a(4) =88a(5) =259a(6) =771a(7) =2284a(8) =6786a(9) =20191a(10) =60189a(11) =179654a(12) =536925a(13) =1606221a(14) =4808601a(15) =14403804a(16) =43163793a(17) =129388755a(18) =387946445a(19) =1163370778a(20) =3489117566a(21) =10465248174a(22) =31391306504a(23) =94164586011a(24) =282474177151a(25) =847381544167