a(n) = binomial(n+1,2)*n!/n!!.

Terms

a(0) =1a(1) =3a(2) =12a(3) =30a(4) =120a(5) =315a(6) =1344a(7) =3780a(8) =17280a(9) =51975a(10) =253440a(11) =810810a(12) =4193280a(13) =14189175a(14) =77414400a(15) =275675400a(16) =1579253760a(17) =5892561675a(18) =35300966400a(19) =137493105750a(20) =858370867200