a(n) = Sum_{d|n} 6^(d-1).

Terms

a(0) =1a(1) =7a(2) =37a(3) =223a(4) =1297a(5) =7819a(6) =46657a(7) =280159a(8) =1679653a(9) =10078999a(10) =60466177a(11) =362805091a(12) =2176782337a(13) =13060740679a(14) =78364165429a(15) =470185264735