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

Terms

a(0) =1a(1) =1a(2) =3a(3) =6a(4) =12a(5) =23a(6) =43a(7) =79a(8) =142a(9) =252a(10) =442a(11) =766a(12) =1316a(13) =2244a(14) =3799a(15) =6393a(16) =10704a(17) =17841a(18) =29618a(19) =49000a(20) =80823a(21) =132964a(22) =218242a(23) =357501a(24) =584608a(25) =954553a(26) =1556575a(27) =2535425a(28) =4125805a(29) =6708143