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

Terms

a(0) =1a(1) =1a(2) =2a(3) =6a(4) =19a(5) =95a(6) =291a(7) =2037a(8) =10203a(9) =71429a(10) =357240a(11) =3929640a(12) =19648533a(13) =255430929a(14) =1788018540a(15) =16092167088a(16) =144829514049