G.f.: (1 / theta_4(x) - 1)^2 / 4, where theta_4() is the Jacobi theta function.

Terms

a(0) =1a(1) =4a(2) =12a(3) =30a(4) =68a(5) =144a(6) =289a(7) =556a(8) =1034a(9) =1868a(10) =3292a(11) =5678a(12) =9608a(13) =15984a(14) =26188a(15) =42314a(16) =67509a(17) =106460a(18) =166090a(19) =256552a(20) =392628a(21) =595696a(22) =896484a(23) =1338894a(24) =1985298a(25) =2923840a(26) =4278448a(27) =6222518a(28) =8997544a(29) =12938368