a(n) = T(n,n), array T as in A050143.

Terms

a(0) =1a(1) =1a(2) =4a(3) =18a(4) =88a(5) =450a(6) =2364a(7) =12642a(8) =68464a(9) =374274a(10) =2060980a(11) =11414898a(12) =63521352a(13) =354870594a(14) =1989102444a(15) =11180805570a(16) =63001648608a(17) =355761664002