A025702 Index of 4^n within sequence of numbers of form 4^i*5^j.

1, 2, 4, 7, 11, 16, 22, 29, 36, 44, 53, 63, 74, 86, 99, 112, 126, 141, 157, 174, 192, 211, 230, 250, 271, 293, 316, 340, 365, 390, 416, 443, 471, 500, 530, 561, 593, 625, 658, 692, 727, 763, 800, 838, 876, 915, 955, 996, 1038, 1081, 1125, 1169, 1214, 1260, 1307, 1355

Offset: 0

Author

David W. Wilson

Keywords

• nonn
• easy

Comments

Positions of zeros in A025650. - R. J. Mathar, Jul 06 2025

Links

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

Crossrefs

Differs from A025717 at a(65).

Programs

PARI

a(n)=my(N=1); n+1+sum(i=1, n, logint(N<<=2, 5)); \\ Charles R Greathouse IV, Jan 11 2018

PARI

first(n)=my(s, N=1/4); vector(n+1, i, s+=logint(N<<=2, 5)+1) \\ Charles R Greathouse IV, Jan 11 2018

Extensions

Offset corrected by Charles R Greathouse IV, Jan 11 2018