Third diagonal of triangle in A046740.

Terms

a(0) =2a(1) =28a(2) =182a(3) =864a(4) =3474a(5) =12660a(6) =43358a(7) =142552a(8) =455930a(9) =1430796a(10) =4431078a(11) =13595664a(12) =41441570a(13) =125732836a(14) =380212142a(15) =1147057800a(16) =3454803594a(17) =10393245180a(18) =31240551350a(19) =93849578560a(20) =281817169202a(21) =846013542228