Index of 7^n within the sequence of the numbers of the form 2^i*7^j.

Terms

a(0) =1a(1) =4a(2) =10a(3) =19a(4) =31a(5) =46a(6) =63a(7) =83a(8) =106a(9) =132a(10) =161a(11) =192a(12) =226a(13) =263a(14) =303a(15) =346a(16) =391a(17) =439a(18) =490a(19) =544a(20) =601a(21) =660a(22) =722a(23) =787a(24) =855a(25) =926a(26) =999a(27) =1075a(28) =1154a(29) =1236