Index of 4^n within the sequence of the numbers of the form 4^i*5^j.

Terms

a(0) =1a(1) =2a(2) =4a(3) =7a(4) =11a(5) =16a(6) =22a(7) =29a(8) =36a(9) =44a(10) =53a(11) =63a(12) =74a(13) =86a(14) =99a(15) =112a(16) =126a(17) =141a(18) =157a(19) =174a(20) =192a(21) =211a(22) =230a(23) =250a(24) =271a(25) =293a(26) =316a(27) =340a(28) =365a(29) =390