a(n) = floor(exp(16/23) * n!).

Terms

a(0) =2a(1) =4a(2) =12a(3) =48a(4) =240a(5) =1443a(6) =10105a(7) =80842a(8) =727580a(9) =7275803a(10) =80033833a(11) =960405999a(12) =12485277998a(13) =174793891980