Index of 5^n within sequence of numbers of form 3^i*5^j.

Terms

a(0) =1a(1) =3a(2) =6a(3) =11a(4) =17a(5) =25a(6) =34a(7) =45a(8) =57a(9) =71a(10) =86a(11) =103a(12) =121a(13) =141a(14) =162a(15) =184a(16) =208a(17) =233a(18) =260a(19) =288a(20) =318a(21) =349a(22) =382a(23) =416a(24) =452a(25) =489a(26) =528a(27) =568a(28) =610a(29) =653