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%I #20 Jul 04 2021 08:43:16
%S 5,17,109,1979,6899,7331,18661,79621,81649
%N Numbers k such that 20^k - 19^k is prime.
%C Terms k > 1000 may correspond to (unproven) strong pseudoprimes.
%H Henri Lifchitz and Renaud Lifchitz, <a href="http://www.primenumbers.net/prptop/searchform.php?form=20%5Ep-19%5Ep">Top probable primes of the form 20^p-19^p</a>
%o (PARI) is(n)=ispseudoprime(20^n-19^n) \\ _Charles R Greathouse IV_, Jun 12 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(7)-a(9) found by _Jean-Louis Charton_ in February and March 2011
%E Edited by _M. F. Hasler_, Sep 16 2013
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