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%I #25 Jan 17 2019 13:44:08
%S 2,6,32,108,408,1286,2268,2328,4284,53558,181182,249010
%N Integers n such that 10^n-21 is a prime number.
%C a(13) > 2.5*10^5. - _Robert Price_, Apr 12 2015
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/aaaba.htm">Near-repdigit numbers of the form AA...AABA</a>.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/9/99979.htm#prime">Prime numbers of the form 99...9979</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%e 6 is a member because 10^6-21 = 1000000-21 = 999979, which is prime.
%Y Cf. A089675, A095714, A092767.
%K more,nonn
%O 1,1
%A Julien Peter Benney (jpbenney(AT)ftml.net), Jun 30 2005
%E Edited by _Ray Chandler_, Dec 23 2010
%E a(10)=53558 from _Robert Price_, Mar 03 2011
%E a(11)-a(12) from Kamada data by _Robert Price_, Apr 12 2015
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