a(n) = psi(2^n-1), where psi = A002322.

Terms

a(0) =1a(1) =2a(2) =6a(3) =4a(4) =30a(5) =6a(6) =126a(7) =16a(8) =72a(9) =30a(10) =88a(11) =12a(12) =8190a(13) =126a(14) =150a(15) =256a(16) =131070a(17) =72a(18) =524286a(19) =120a(20) =1008a(21) =2728a(22) =178480a(23) =240a(24) =1800a(25) =8190a(26) =262656a(27) =1008a(28) =39672a(29) =1650