a(0) = a(1) = 1; a(n) = n * Sum_{k=0..n-2} a(k) * a(n-k-2).

Terms

a(0) =1a(1) =1a(2) =2a(3) =6a(4) =20a(5) =80a(6) =336a(7) =1568a(8) =7584a(9) =39312a(10) =210080a(11) =1180256a(12) =6813312a(13) =40890304a(14) =251528704a(15) =1597332480a(16) =10376040448a(17) =69259146752a(18) =472084038144