a(n) = A002531(n)*A002531(n+1).

Terms

a(0) =1a(1) =2a(2) =10a(3) =35a(4) =133a(5) =494a(6) =1846a(7) =6887a(8) =25705a(9) =95930a(10) =358018a(11) =1336139a(12) =4986541a(13) =18610022a(14) =69453550a(15) =259204175a(16) =967363153a(17) =3610248434a(18) =13473630586a(19) =50284273907a(20) =187663465045a(21) =700369586270