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

Terms

a(0) =1a(1) =3a(2) =5a(3) =8a(4) =12a(5) =18a(6) =28a(7) =42a(8) =65a(9) =103a(10) =160a(11) =252a(12) =404a(13) =644a(14) =1026a(15) =1648a(16) =2654a(17) =4269a(18) =6873a(19) =11086a(20) =17898a(21) =28902a(22) =46681a(23) =75427a(24) =121920a(25) =197116a(26) =318731a(27) =515425a(28) =833593a(29) =1348316