a(n) = sigma^n(n), where sigma^n denotes functional iteration.

Terms

a(0) =1a(1) =4a(2) =8a(3) =24a(4) =120a(5) =1170a(6) =1512a(7) =15748a(8) =15748a(9) =61440a(10) =196584a(11) =1572840a(12) =1098240a(13) =15605760a(14) =50328576a(15) =7074432a(16) =355532800a(17) =2143289344a(18) =13815410400a(19) =224319234048a(20) =33541980160a(21) =109760857440