a(n) = [ C(2n,n)/n^2 ].

Terms

a(0) =2a(1) =1a(2) =2a(3) =4a(4) =10a(5) =25a(6) =70a(7) =201a(8) =600a(9) =1847a(10) =5830a(11) =18778a(12) =61542a(13) =204676a(14) =689411a(15) =2347970a(16) =8074762a(17) =28009676a(18) =97909318a(19) =344616322a(20) =1220539397a(21) =4347311908a(22) =15564141262a(23) =55985423060a(24) =202256970300a(25) =733607297260