a(n) = Sum_{d|n} phi(n/d)^(n-d+1).

Terms

a(0) =1a(1) =2a(2) =9a(3) =18a(4) =1025a(5) =98a(6) =279937a(7) =65666a(8) =10077825a(9) =1310722a(10) =100000000001a(11) =16780802a(13) =91424858114