G.f. satisfies: A(x) = (1+x+x^2)^3 * A(x^2)^2.

Terms

a(0) =1a(1) =3a(2) =12a(3) =25a(4) =75a(5) =144a(6) =357a(7) =615a(8) =1380a(9) =2285a(10) =4767a(11) =7488a(12) =14817a(13) =22707a(14) =43068a(15) =63769a(16) =116667a(17) =169584a(18) =301589a(19) =427815a(20) =741396a(21) =1037149a(22) =1761087a(23) =2418432a(24) =4025153a(25) =5465955a(26) =8956716a(27) =11986009a(28) =19330347a(29) =25633296