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%I #31 Nov 10 2019 01:33:19
%S 10907,499211
%N Numbers k such that (64*10^(k-1) + 53)/9 is a depression prime.
%C Prime versus probable prime status and proofs are given in the author's table.
%C Searched up to k=1200000 by _Serge Batalov_, Mar 02 2015
%D C. Caldwell and H. Dubner, "Journal of Recreational Mathematics", Volume 28, No. 1, 1996-97, pp. 1-9.
%H Patrick De Geest, <a href="http://www.worldofnumbers.com/deplat.htm#pdp717">PDP Reference Table - 717</a>.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/7/71117.htm#prime">Prime numbers of the form 711...117</a>.
%H Tanya Khovanova, <a href="http://www.tanyakhovanova.com/RecursiveSequences/NonRecursions.html">Non Recursions</a>
%H C. Rivera and J.C. Rosa, <a href="http://www.primepuzzles.net/puzzles/puzz_197.htm">Puzzle 197. Always composite numbers?</a>
%e a(1)=10907 -> (64*10^(10907-1) + 53)/9 = 7111...1117 or 10905 1's surrounded by two 7's.
%Y Cf. A082697-A082720.
%K nonn,bref,base,hard,more
%O 1,1
%A _Patrick De Geest_, Apr 13 2003
%E Edited by _Ray Chandler_, Nov 05 2014
%E Additional PRP term 499211 by _Serge Batalov_, Mar 01 2015
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