%I #28 Jun 28 2021 12:05:41
%S 1,30,329,3319,33216,332190,3321925,33219278,332192807,3321928092,
%T 33219280946,332192809486,3321928094885,33219280948871,
%U 332192809488733,3321928094887360,33219280948873621,332192809488736232,3321928094887362345,33219280948873623476
%N a(n) is the least integer k such that 2^k - 1 has at least 10^n digits.
%H Alois P. Heinz, <a href="/A227689/b227689.txt">Table of n, a(n) for n = 0..1000</a>
%H Wikipedia, <a href="http://en.wikipedia.org/wiki/Great_Internet_Mersenne_Prime_Search">Great Internet Mersenne Prime Search</a>
%F a(n) = ceiling(log_2(10^(10^n-1)+1)).
%F Limit_{n -> oo} a(n)/10^n = log_2(10) = A020862. - _Alois P. Heinz_, Jun 28 2021
%e For n = 2, A000225(328) has 99 digits and A000225(329) has 100 digits, so a(2) = 329.
%o (PARI) a(n) = ceil(log(10^(10^n-1)+1)/log(2)); \\ _Michel Marcus_, Jun 28 2021
%Y See A000225, A020862, A034887.
%K nonn,base
%O 0,2
%A _Olivier de Mouzon_, Jul 19 2013
%E a(7)-a(19) from _Alois P. Heinz_, Jun 28 2021