a(1) = 1; a(n+1) = Sum_{d|n} phi(d) * a(d).

Terms

a(0) =1a(1) =1a(2) =2a(3) =5a(4) =12a(5) =49a(6) =104a(7) =625a(8) =2512a(9) =15077a(10) =60358a(11) =603581a(12) =2414438a(13) =28973257a(14) =173840168a(15) =1390721397a(16) =11125773688a(17) =178012379009