A065718 - OEIS

%I #26 Apr 25 2020 13:33:01

%S 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,

%T 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,

%U 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

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

%H Harry J. Smith, <a href="/A065718/b065718.txt">Table of n, a(n) for n = 0..1000</a>

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

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

%o (PARI) a(n) = #select(x->(x==7), digits(2^n)); \\ _Michel Marcus_, Jun 15 2018

%o (Python)

%o def A065718(n):

%o     return str(2**n).count('7') # _Chai Wah Wu_, Feb 14 2020

%Y 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.

%K nonn,base

%O 0,25

%A _Benoit Cloitre_, Dec 04 2001

%E More terms from _Robert G. Wilson v_, Dec 07 2001