a(n) = (1/(6*n)) * Sum_{d|n} 7^(n/d-1) * phi(7*d).

Terms

a(0) =1a(1) =4a(2) =17a(3) =88a(4) =481a(5) =2812a(6) =16808a(7) =102988a(8) =640545a(9) =4035604a(10) =25679569a(11) =164778696a(12) =1064714401a(13) =6920652008a(14) =45214871857a(15) =296722645888