a(n) = Sum_{d|n} phi(d)^n.

Terms

a(0) =1a(1) =2a(2) =9a(3) =18a(4) =1025a(5) =130a(6) =279937a(7) =65794a(8) =10078209a(9) =2097154a(10) =100000000001a(11) =16789506a(13) =156728328194