G.f.: Product_{j>=1} (1+x^j)^(3^j).

Terms

a(0) =1a(1) =3a(2) =12a(3) =55a(4) =225a(5) =927a(6) =3729a(7) =14787a(8) =57888a(9) =224220a(10) =860022a(11) =3270744a(12) =12343899a(13) =46264257a(14) =172305837a(15) =638039136a(16) =2350109736a(17) =8613851832a(18) =31428857611a(19) =114187160631a(20) =413222547846