G.f.: 1 / AGM((1 - 3*x)^2, (1 + x)^2).

Terms

a(0) =1a(1) =2a(2) =3a(3) =4a(4) =9a(5) =30a(6) =91a(7) =232a(8) =549a(9) =1378a(10) =3839a(11) =11100a(12) =31301a(13) =85694a(14) =234207a(15) =653328a(16) =1856829a(17) =5300010a(18) =15062839a(19) =42702596a(20) =121448901a(21) =347414166a(22) =997886671a(23) =2870139480a(24) =8257776521a(25) =23782773242a(26) =68627659563a(27) =198437633884a(28) =574654851209