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

Terms

a(0) =1a(1) =3a(2) =7a(3) =13a(4) =21a(5) =30a(6) =41a(7) =54a(8) =69a(9) =85a(10) =103a(11) =123a(12) =145a(13) =169a(14) =194a(15) =221a(16) =250a(17) =281a(18) =313a(19) =347a(20) =383a(21) =421a(22) =460a(23) =501a(24) =544a(25) =589a(26) =636a(27) =684a(28) =734a(29) =786