A025706 Index of 5^n within sequence of numbers of form 4^i*5^j.

1, 3, 6, 10, 15, 21, 28, 37, 47, 58, 70, 83, 97, 113, 130, 148, 167, 187, 208, 231, 255, 280, 306, 333, 361, 391, 422, 454, 487, 521, 556, 592, 630, 669, 709, 750, 792, 835, 880, 926, 973, 1021, 1070, 1120, 1172, 1225, 1279, 1334, 1390, 1447, 1506, 1566, 1627, 1689

Offset: 0

Author

David W. Wilson

Keywords

• nonn
• easy

Comments

Positions of zeros in A025646. - R. J. Mathar, Jul 06 2025

Links

• Charles R Greathouse IV, Table of n, a(n) for n = 0..10000

Crossrefs

Cf. A025646. Differs from A025730 at a(56).

Programs

PARI

a(n)=my(N=1); n+1+sum(i=1, n, logint(N*=5, 4)); \\ Charles R Greathouse IV, Jan 11 2018

PARI

first(n)=my(s, N=1/5); vector(n+1, i, s+=logint(N*=5, 4)+1) \\ Charles R Greathouse IV, Jan 11 2018

Extensions

Offset corrected by Charles R Greathouse IV, Jan 11 2018