(1/2)*A075998.

Terms

a(0) =1a(1) =3a(2) =4a(3) =30a(4) =24a(5) =168a(6) =288a(7) =1080a(8) =1920a(9) =12672a(10) =8640a(11) =131040a(12) =161280a(13) =414720a(14) =774144a(15) =10575360a(16) =8294400a(17) =36771840a(18) =69672960a(19) =383201280a(20) =729907200a(21) =4578508800a(22) =2043740160a(23) =45287424000a(24) =65213890560a(25) =180592312320a(26) =347807416320