a(n) = GCD(reverse(n!), reverse((n+1)!)).

Terms

a(0) =1a(1) =2a(2) =6a(3) =21a(4) =3a(5) =27a(6) =9a(7) =9a(8) =88263a(9) =9a(10) =99a(11) =594a(12) =198a(13) =99a(14) =99a(15) =99a(16) =99a(17) =99a(18) =99a(19) =9009a(20) =99a(21) =99a(22) =198a(23) =99a(24) =99a(25) =297a(26) =1089a(27) =99a(28) =198a(29) =198