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A308449 Numbers k such that 25*10^(2*k) + 5*10^k + 1 are prime. 1

%I

%S 0,1,2,14,174,448

%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

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Last modified February 3 10:55 EST 2023. Contains 360034 sequences. (Running on oeis4.)