G.f. satisfies A(x) = 1 + x*A(x)^2/(1 - x^2*A(x)^4).

Terms

a(0) =1a(1) =1a(2) =2a(3) =6a(4) =22a(5) =88a(6) =370a(7) =1613a(8) =7230a(9) =33117a(10) =154330a(11) =729369a(12) =3487470a(13) =16840346a(14) =82007012a(15) =402269702a(16) =1985867630a(17) =9858739759a(18) =49187798158a(19) =246506563980