a(n) = T(2*n, n-1), where T is given by A026552.

Terms

a(0) =1a(1) =3a(2) =12a(3) =45a(4) =180a(5) =721a(6) =2940a(7) =12069a(8) =49935a(9) =207691a(10) =867900a(11) =3640429a(12) =15319395a(13) =64643580a(14) =273431408a(15) =1158988141a(16) =4921651521a(17) =20934115963a(18) =89173404140a(19) =380355072153