a(n) = (n+1)!/LCM{1,3,6,...,C(n+1,2)}.

Terms

a(0) =2a(1) =2a(2) =4a(3) =4a(4) =24a(5) =24a(6) =96a(7) =288a(8) =2880a(9) =2880a(10) =34560a(11) =34560a(12) =483840a(13) =7257600a(14) =58060800a(15) =58060800a(16) =1045094400a(17) =1045094400a(18) =20901888000a(19) =438939648000