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

%I A065718 #26 Apr 25 2020 13:33:01
%S A065718 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 A065718 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 A065718 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 A065718 Number of 7's in decimal expansion of 2^n.
%H A065718 Harry J. Smith, <a href="/A065718/b065718.txt">Table of n, a(n) for n = 0..1000</a>
%e A065718 2^15 = 32768 so a(15)=1.
%t A065718 Table[ Count[ IntegerDigits[2^n], 7], {n, 0, 100} ]
%o A065718 (PARI) a(n) = #select(x->(x==7), digits(2^n)); \\ _Michel Marcus_, Jun 15 2018
%o A065718 (Python)
%o A065718 def A065718(n):
%o A065718     return str(2**n).count('7') # _Chai Wah Wu_, Feb 14 2020
%Y A065718 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 A065718 nonn,base
%O A065718 0,25
%A A065718 _Benoit Cloitre_, Dec 04 2001
%E A065718 More terms from _Robert G. Wilson v_, Dec 07 2001