Index of 7^n within the sequence of the numbers of the form 5^i*7^j.

Terms

a(0) =1a(1) =3a(2) =6a(3) =10a(4) =15a(5) =22a(6) =30a(7) =39a(8) =49a(9) =60a(10) =73a(11) =87a(12) =102a(13) =118a(14) =135a(15) =154a(16) =174a(17) =195a(18) =217a(19) =240a(20) =265a(21) =291a(22) =318a(23) =346a(24) =376a(25) =407a(26) =439a(27) =472a(28) =506a(29) =542