A065716 - OEIS

A065716

Number of 5's in decimal expansion of 2^n.

0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 0, 0, 0, 0, 0, 0, 2, 0, 0, 1, 1, 1, 0, 0, 0, 2, 0, 0, 2, 1, 0, 0, 0, 2, 0, 1, 0, 1, 0, 3, 1, 4, 1, 0, 1, 1, 0, 2, 1, 2, 1, 2, 2, 1, 1, 0, 2, 4, 1, 2, 2, 2, 0, 2, 2, 0, 0, 3, 5, 5, 1, 0, 1, 1, 3, 2, 4, 3, 3, 2, 1, 3, 3, 2, 1, 3, 4, 4, 3, 1, 1, 3, 4, 1, 3, 2, 4, 5, 5, 2, 2

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OFFSET

0,17

LINKS

Harry J. Smith, Table of n, a(n) for n = 0..1000

EXAMPLE

2^8 = 256 so a(8)=1.

MATHEMATICA

Table[ Count[ IntegerDigits[2^n], 5], {n, 0, 100} ]

DigitCount[#, 10, 5]&/@(2^Range[0, 100]) (* Harvey P. Dale, Nov 13 2021 *)

PROG

(PARI) a(n) = #select(x->(x==5), digits(2^n)); \\ Michel Marcus, Jun 15 2018