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%I #31 Apr 17 2023 18:36:07
%S 0,1,2,14,174,448,16562
%N Numbers k such that 25*10^(2*k) + 5*10^k + 1 are prime.
%e 31 is prime ==> a(1) = 0.
%e 2551 is prime ==> a(2) = 1.
%e 250501 is prime ==> a(3) = 2.
%e 25005001 = 7 * 79 * 103 * 439.
%e 2500050001 = 19 * 2269 * 57991.
%e 250000500001 = 7 * 103561 * 344863.
%o (PARI) for(k=0, 1e3, if(ispseudoprime(25*100^k+5*10^k+1), print1(k", ")))
%Y Cf. A309738.
%K nonn,more,base
%O 1,3
%A _Seiichi Manyama_, Aug 15 2019
%E a(7) from _Michael S. Branicky_, Apr 17 2023
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