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Numbers k such that 2 is the first digit of 2^k.
8

%I #29 Jan 27 2021 05:34:44

%S 1,8,11,18,21,28,31,38,41,48,51,58,61,68,71,81,91,101,104,111,114,121,

%T 124,131,134,141,144,151,154,161,164,171,174,184,194,197,204,207,214,

%U 217,224,227,234,237,244,247,254,257,264,267,277,287,297,300,307,310

%N Numbers k such that 2 is the first digit of 2^k.

%C The asymptotic density of this sequence is log_10(3/2) = 0.176091... (A154580 - 1). - _Amiram Eldar_, Jan 27 2021

%H Muniru A Asiru, <a href="/A067469/b067469.txt">Table of n, a(n) for n = 1..5000</a>

%t Select[Range@ 310, First@ IntegerDigits[2^#] == 2 &] (* _Michael De Vlieger_, Oct 15 2018 *)

%o (PARI) is(n)=digits(2^n)[1]==2 \\ _Charles R Greathouse IV_, Jul 29 2013

%o (GAP) Filtered([1..310],n->ListOfDigits(2^n)[1]=2); # _Muniru A Asiru_, Oct 14 2018

%Y Cf. A067480, A154580.

%K nonn,base

%O 1,2

%A _Benoit Cloitre_, Feb 22 2002