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

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =3a(5) =11a(6) =31a(7) =71a(8) =157a(9) =397a(10) =1141a(11) =3301a(12) =9079a(13) =24207a(14) =65339a(15) =182131a(16) =517307a(17) =1467067a(18) =4128859a(19) =11606683a(20) =32835433a(21) =93588097a(22) =267745149a(23) =766331573a(24) =2193690811a(25) =6289737611a(26) =18081071971a(27) =52099389811a(28) =150344751721a(29) =434277817873