A product of quotients of factorials.

Terms

a(0) =1a(1) =2a(2) =6a(3) =12a(4) =60a(5) =120a(6) =840a(7) =7560a(8) =15120a(9) =110880a(10) =166320a(11) =1441440a(12) =2882880a(13) =10810800a(14) =43243200a(15) =183783600a(16) =367567200a(17) =2793510720a(18) =6983776800a(19) =58663725120a(20) =117327450240a(21) =299836817280