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

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =2a(5) =5a(6) =15a(7) =51a(8) =191a(9) =773a(10) =3338a(11) =15243a(12) =73131a(13) =366815a(14) =1916260a(15) =10394665a(16) =58404853a(17) =339223859a(18) =2033188222a(19) =12556915219a(20) =79807729238a(21) =521399203037