a(n) = A026998(2*n, n+4).

Terms

a(0) =1a(1) =64a(2) =901a(3) =6821a(4) =36425a(5) =155793a(6) =573382a(7) =1899933a(8) =5844446a(9) =17056486a(10) =47974934a(11) =131553646a(12) =354615679a(13) =945220982a(14) =2501450971a(15) =6590435731a(16) =17316698039a(17) =45428211431a(18) =119066290172a(19) =311909267867a(20) =816853717452