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%I #24 Jul 04 2021 07:52:06
%S 2,3,7,11,19,29,401,709,2531,15787,66949,282493
%N Numbers k such that 10^k - 9^k is prime.
%C Terms > 1000 are often only strong pseudoprimes.
%C All terms are prime. - _Alexander Adamchuk_, Apr 27 2008
%H Henri & Renaud Lifchitz, <a href="http://www.primenumbers.net/prptop/searchform.php?form=10%5Ex-9%5Ex&action=Search">PRP Records</a>.
%e 10^2 - 9^2 = 100 - 81 = 19, which is prime, hence 2 is in the sequence.
%e 10^3 - 9^3 = 1000 - 729 = 271, which is prime, hence 3 is in the sequence.
%e 10^4 - 9^4 = 10000 - 6561 = 3439 = 19 * 181, which is not prime, hence 4 is not in the sequence.
%t Select[Range[1000], PrimeQ[10^# - 9^#] &] (* _Alonso del Arte_, Sep 06 2013 *)
%o (PARI) is(n)=ispseudoprime(10^n-9^n) \\ _Charles R Greathouse IV_, Feb 20 2017
%Y Cf. A000043, A057468, A059801, A059802, A059803 (9^n-8^n is prime), A062572-A062666.
%Y Cf. A016189 = 10^n - 9^n, and A199819 (primes of this form).
%K nonn,hard
%O 1,1
%A _Mike Oakes_, May 18 2001, May 19 2001
%E Three more terms 15787, 66949 and 282493 found by _Jean-Louis Charton_ in 2004 and 2007
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