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

Terms

a(0) =0a(1) =6a(2) =42a(3) =228a(4) =1344a(5) =7800a(6) =46956a(7) =279972a(8) =1681008a(9) =10078164a(10) =60474120a(11) =362797116a(12) =2176832112a(13) =13060694088a(14) =78364444284a(15) =470185001040