a(n) = (2n+1)!/n!^2.

Terms

a(0) =1a(1) =6a(2) =30a(3) =140a(4) =630a(5) =2772a(6) =12012a(7) =51480a(8) =218790a(9) =923780a(10) =3879876a(11) =16224936a(12) =67603900a(13) =280816200a(14) =1163381400a(15) =4808643120a(16) =19835652870a(17) =81676217700a(18) =335780006100