Index of 7^n within the sequence of the numbers of the form 4^i*7^j.

Terms

a(0) =1a(1) =3a(2) =6a(3) =11a(4) =17a(5) =25a(6) =34a(7) =44a(8) =56a(9) =69a(10) =84a(11) =100a(12) =117a(13) =136a(14) =156a(15) =178a(16) =201a(17) =225a(18) =251a(19) =278a(20) =307a(21) =337a(22) =368a(23) =401a(24) =435a(25) =471a(26) =508a(27) =546a(28) =586a(29) =627