Partial sums of A013999.

Terms

a(0) =1a(1) =2a(2) =4a(3) =12a(4) =54a(5) =312a(6) =2136a(7) =16800a(8) =149160a(9) =1475280a(10) =16081920a(11) =191530080a(12) =2473999920a(13) =34446303360a(14) =514240110720