G.f. A(x) satisfies 1 - x = Sum_{n=-oo..+oo} (x^(2*n) - A(x))^n.

Terms

a(0) =1a(1) =3a(2) =5a(3) =11a(4) =20a(5) =38a(6) =67a(7) =119a(8) =211a(9) =398a(10) =830a(11) =1940a(12) =4902a(13) =12784a(14) =33165a(15) =84136a(16) =207240a(17) =495964a(18) =1157767a(19) =2654461a(20) =6029627a(21) =13704225a(22) =31463620a(23) =73498385a(24) =175220708a(25) =425631952a(26) =1048102141a(27) =2599306042a(28) =6453178098a(29) =15967452038