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A271360 Numbers k such that 5*10^k + 87 is prime.

%I A271360 #14 Jun 06 2024 23:24:08
%S A271360 1,2,3,4,6,16,38,39,46,61,62,100,116,152,190,298,472,642,688,1676,
%T A271360 1971,2338,4389,4520,4986,7765,9428,10820,18984,19797,35360,42146,
%U A271360 168807
%N A271360 Numbers k such that 5*10^k + 87 is prime.
%C A271360 For k > 1, numbers k such that the digit 5 followed by k-2 occurrences of the digit 0 followed by the digits 87 is prime (see Example section).
%C A271360 a(34) > 2*10^5.
%H A271360 Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/prime_difficulty.txt">Search for 50w87</a>.
%e A271360 3 is in this sequence because 5*10^3 + 87 = 5087 is prime.
%e A271360 Initial terms and associated primes:
%e A271360 a(1) = 1, 137;
%e A271360 a(2) = 2, 587;
%e A271360 a(3) = 3, 5087;
%e A271360 a(4) = 4, 50087;
%e A271360 a(5) = 6, 5000087, etc.
%t A271360 Select[Range[0, 100000], PrimeQ[5*10^# + 87] &]
%o A271360 (PARI) is(n)=ispseudoprime(5*10^n + 87) \\ _Charles R Greathouse IV_, Jun 13 2017
%Y A271360 Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K A271360 nonn,more
%O A271360 1,2
%A A271360 _Robert Price_, Apr 05 2016
%E A271360 a(33) from _Robert Price_, Feb 24 2019