Index of 7^n within the sequence of the numbers of the form 6^i*7^j.

Terms

a(0) =1a(1) =3a(2) =6a(3) =10a(4) =15a(5) =21a(6) =28a(7) =36a(8) =45a(9) =55a(10) =66a(11) =78a(12) =92a(13) =107a(14) =123a(15) =140a(16) =158a(17) =177a(18) =197a(19) =218a(20) =240a(21) =263a(22) =287a(23) =312a(24) =339a(25) =367a(26) =396a(27) =426a(28) =457a(29) =489