a(n) = 2*(3*n)! / ((2*n+1)!*(n+1)!).

Terms

a(0) =2a(1) =1a(2) =2a(3) =6a(4) =22a(5) =91a(6) =408a(7) =1938a(8) =9614a(9) =49335a(10) =260130a(11) =1402440a(12) =7702632a(13) =42975796a(14) =243035536a(15) =1390594458a(16) =8038677054a(17) =46892282815a(18) =275750636070