A065718 Number of 7's in decimal expansion of 2^n.

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

Offset: 0

Author

Benoit Cloitre, Dec 04 2001

Keywords

• nonn
• base

Examples

			2^15 = 32768 so a(15)=1.
		

Links

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

Crossrefs

Cf. 0's A027870, 1's A065712, 2's A065710, 3's A065714, 4's A065715, 5's A065716, 6's A065717, 8's A065719, 9's A065744.

Programs

Mathematica

Table[ Count[ IntegerDigits[2^n], 7], {n, 0, 100} ]

PARI

a(n) = #select(x->(x==7), digits(2^n)); \\ Michel Marcus, Jun 15 2018

Python

def A065718(n):
    return str(2**n).count('7') # Chai Wah Wu, Feb 14 2020

Extensions

More terms from Robert G. Wilson v, Dec 07 2001