Generalized Catalan numbers C(2,2; n).

Terms

a(0) =1a(1) =1a(2) =4a(3) =28a(4) =256a(5) =2704a(6) =31168a(7) =380608a(8) =4840960a(9) =63458560a(10) =851399680a(11) =11635096576a(12) =161396604928