a(n) = n!/(floor(n/2)!*floor(n/3)!*floor(n/6)!).

Terms

a(0) =1a(1) =1a(2) =2a(3) =6a(4) =12a(5) =60a(6) =60a(7) =420a(8) =840a(9) =2520a(10) =5040a(11) =55440a(12) =13860a(13) =180180a(14) =360360a(15) =1081080a(16) =2162160a(17) =36756720a(18) =4084080a(19) =77597520a(20) =155195040a(21) =465585120a(22) =931170240a(23) =21416915520a(24) =1338557220a(25) =33463930500a(26) =66927861000a(27) =200783583000a(28) =401567166000a(30) =465817912560