a(n) = 6*(n+1)!/((3+floor(n/2))*(floor(n/2)!)^2).

Terms

a(0) =2a(1) =4a(2) =9a(3) =36a(4) =36a(5) =216a(6) =140a(7) =1120a(8) =540a(9) =5400a(10) =2079a(11) =24948a(12) =8008a(13) =112112a(14) =30888a(15) =494208a(16) =119340a(17) =2148120a(18) =461890a(19) =9237800a(20) =1790712a(21) =39395664a(22) =6953544a(23) =166885056a(24) =27041560a(25) =703080560a(26) =105306075a(27) =2948570100a(28) =410605200a(29) =12318156000