Index of 9^n within the sequence of the numbers of the form 7^i*9^j.

Terms

a(0) =1a(1) =3a(2) =6a(3) =10a(4) =15a(5) =21a(6) =28a(7) =36a(8) =46a(9) =57a(10) =69a(11) =82a(12) =96a(13) =111a(14) =127a(15) =144a(16) =163a(17) =183a(18) =204a(19) =226a(20) =249a(21) =273a(22) =298a(23) =324a(24) =352a(25) =381a(26) =411a(27) =442a(28) =474a(29) =507