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A278697 Primes p such that every suffix of the base-5 representation of p is a prime.

%I A278697 #16 Jan 22 2020 17:26:52
%S A278697 2,3,7,13,17,23,53,67,73,103,107,113,127,257,263,317,353,503,523,607,
%T A278697 613,1303,1567,1753,1877,2503,3023,6257,6263,6317,6323,6353,6857,6863,
%U A278697 7817,8753,9377,12503,12517,12553,12613,12757,12763,12853,13003,31253,31267,31357,31513,31567
%N A278697 Primes p such that every suffix of the base-5 representation of p is a prime.
%H A278697 Robert Israel, <a href="/A278697/b278697.txt">Table of n, a(n) for n = 1..10000</a>
%e A278697 17=32_5 is in the sequence since it and its base-5 suffix (2_5=2) are primes.
%e A278697 113=423_5 is in the sequence since it and each of its base-5 suffixes (23_5=13 and 3_5=3) are prime.
%p A278697 F[1]:= [2,3]:
%p A278697 for m from 2 while nops(F[m-1]) < 100 do
%p A278697   F[m]:= [op(F[m-1]),op(select(isprime, [seq(seq(i*5^
%p A278697 (m-1)+x,x=F[m-1]),i=1..4)]))]
%p A278697 od:
%p A278697 F[m-1]; # _Robert Israel_, Jan 22 2020
%o A278697 (PARI) isok(n) = {if (isprime(n), pp = 5^logint(n, 5); while (isprime(n % pp) && (pp != 1), pp = pp/5); pp == 1;);} \\ _Michel Marcus_, Nov 26 2016
%Y A278697 Cf. A033664 is in base 10.
%K A278697 nonn,base
%O A278697 1,1
%A A278697 _Randy L. Ekl_, Nov 26 2016