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%I #24 Apr 10 2024 03:31:39
%S 1,5,10,12,24,48,960,2217,13045,13140
%N Numbers k such that 10^(2k)/2 - 10^k + 1 is prime.
%C Numbers of this form divide 10^(4k)+4.
%e 41, 4999900001, 49999999990000000001, and 499999999999000000000001 are prime, while 4901=13^2*29, 499001=17*149*197, etc.
%t ParallelMap[If[PrimeQ[10^(2 #)/2 - 10^# + 1], #, Nothing] &, Range@ 4000] (* _Robert G. Wilson v_, Dec 13 2017 *)
%o (PARI) isok(k) = isprime(10^(2*k)/2-10^k+1); \\ _Michel Marcus_, Dec 13 2017
%Y See A296446 for 10^(2k)/2 + 10^k + 1.
%K nonn,more
%O 1,2
%A _Patrick A. Thomas_, Dec 13 2017
%E a(7) from _Michel Marcus_, Dec 13 2017
%E a(8) from _Robert G. Wilson v_, Dec 13 2017
%E a(9)-a(10) from _Michael S. Branicky_, Apr 09 2024
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