a(n) = phi(3^n+1), where phi is Euler's totient function (A000010).

Terms

a(0) =1a(1) =2a(2) =4a(3) =12a(4) =40a(5) =120a(6) =288a(7) =1092a(8) =3072a(9) =7776a(10) =23600a(11) =87120a(12) =259200a(13) =797160a(14) =1847104a(15) =5832000a(16) =21523360a(17) =63672480a(18) =152845056a(19) =580921200a(20) =1700870400a(21) =4368821184a(22) =12550120000a(23) =47071589412a(24) =130459631616a(25) =413939700000a(26) =997562438080