G.f.: Product_{k>=1} (1+x^k)^(2*k+1).

Terms

a(0) =1a(1) =3a(2) =8a(3) =23a(4) =55a(5) =129a(6) =291a(7) =627a(8) =1317a(9) =2697a(10) =5398a(11) =10589a(12) =20421a(13) =38743a(14) =72452a(15) =133724a(16) =243792a(17) =439496a(18) =784070a(19) =1385195a(20) =2424971a(21) =4209094a(22) =7247141a(23) =12383496a(24) =21008559a(25) =35398548a(26) =59259781a(27) =98595110a(28) =163077878a(29) =268221706