Leading diagonal of triangle in A211321.

Terms

a(0) =1a(1) =1a(2) =2a(3) =6a(4) =24a(5) =116a(6) =632a(7) =3720a(8) =23072a(9) =148528a(10) =983072a(11) =6647776a(12) =45727616a(13) =318947136a(14) =2250473344a(15) =16034726016a(16) =115204555264a(17) =833701020416