a(n) = 2^(2*n-2) - Catalan(n).

Terms

a(0) =0a(1) =2a(2) =11a(3) =50a(4) =214a(5) =892a(6) =3667a(7) =14954a(8) =60674a(9) =245348a(10) =989790a(11) =3986292a(12) =16034316a(13) =64434424a(14) =258740611a(15) =1038384154a(16) =4165322506a(17) =16702230484a(18) =66952213546a(19) =268313786524