A025693 Index of 2^n within sequence of numbers of form 2^i*7^j.

1, 2, 3, 5, 7, 9, 12, 15, 18, 22, 26, 30, 35, 40, 45, 51, 57, 64, 71, 78, 86, 94, 102, 111, 120, 129, 139, 149, 159, 170, 181, 193, 205, 217, 230, 243, 256, 270, 284, 298, 313, 328, 343, 359, 375, 392, 409, 426, 444, 462, 480, 499, 518, 537, 557, 577, 597, 618, 639, 661, 683

Offset: 0

Author

David W. Wilson

Keywords

• nonn
• easy

Comments

Positions of zeros in A025664. - R. J. Mathar, Jul 06 2025

Links

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

Crossrefs

Cf. A003591.

Programs

PARI

a(n)=my(N=7<Charles R Greathouse IV, Jun 28 2011

PARI

a(n)=my(N=1); n+1+sum(i=1, n, logint(N<<=1, 7)); \\ Charles R Greathouse IV, Jan 11 2018

PARI

first(n)=my(s, N=1/2); vector(n+1, i, s+=logint(N<<=1, 7)+1) \\ Charles R Greathouse IV, Jan 11 2018

Formula

a(n) ~ kn^2 + O(n) with k = log(7)/log(2) - log(7)^2/log(2)^2. - Charles R Greathouse IV, Jun 28 2011

Extensions

An incorrect g.f. was deleted by N. J. A. Sloane, Sep 16 2009