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%I #44 Jul 04 2021 07:53:06
%S 2,19,41,43,337,479,9127,37549,44017,59971,128327,176191,193601
%N Numbers k such that 21^k - 20^k is prime.
%C Terms greater than 1000 may correspond to (unproven) strong pseudoprimes.
%H <a href="http://www.primenumbers.net/prptop/searchform.php?form=21%5Ep-20%5Ep">Top probable primes of the form 21^p-20^p</a>
%o (PARI) is(n)=ispseudoprime(21^n-20^n) \\ _Charles R Greathouse IV_, Jun 13 2017
%Y Cf. A000043, A057468, A059801, A059802, A062572-A062666.
%K nonn,hard
%O 1,1
%A _Mike Oakes_, May 18 2001, May 19 2001
%E a(8) from _Jean-Louis Charton_, Feb 29 2012
%E a(9) and a(10) from _Robert Price_, May 30 2012
%E Edited by _M. F. Hasler_, Sep 16 2013
%E a(11) added by _Jean-Louis Charton_, Nov 24 2014
%E a(12) added by _Jean-Louis Charton_, Feb 05 2015
%E a(13) added by _Jean-Louis Charton_, Feb 18 2015
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