Index of 3^n within the sequence of the numbers of the form 3^i*7^j.

Terms

a(0) =1a(1) =2a(2) =4a(3) =6a(4) =9a(5) =12a(6) =16a(7) =20a(8) =25a(9) =31a(10) =37a(11) =44a(12) =51a(13) =59a(14) =67a(15) =76a(16) =86a(17) =96a(18) =107a(19) =118a(20) =130a(21) =142a(22) =155a(23) =168a(24) =182a(25) =197a(26) =212a(27) =228a(28) =244a(29) =261