G.f. satisfies: A(x) = exp( Sum_{n>=1} (A(x)^n + A(-x)^n) * x^n/n ).

Terms

a(0) =1a(1) =2a(2) =3a(3) =10a(4) =27a(5) =112a(6) =336a(7) =1490a(8) =4791a(9) =22138a(10) =74079a(11) =351288a(12) =1207620a(13) =5831208a(14) =20436516a(15) =100004994a(16) =355610367a(17) =1758044950a(18) =6322608561a(19) =31511387450a(20) =114359284515a(21) =573713781760