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A271640 Numbers k such that 3*10^k + 73 is prime.

%I A271640 #24 Jun 06 2024 23:23:53
%S A271640 1,2,5,6,13,37,50,55,71,89,217,352,355,398,449,668,742,870,1360,1579,
%T A271640 2848,3774,5039,5051,6134,6824,7255,12586,17106,27502,30581,33817,
%U A271640 97399,170800,172219,177872
%N A271640 Numbers k such that 3*10^k + 73 is prime.
%C A271640 For k > 1, numbers k such that the digit 3 followed by k-2 occurrences of the digit 0 followed by the digits 73 is prime (see Example section).
%C A271640 a(37) > 2*10^5.
%H A271640 Makoto Kamada, <a href="https://stdkmd.net/nrr">Factorization of near-repdigit-related numbers</a>.
%H A271640 Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/prime_difficulty.txt">Search for 30w73</a>.
%e A271640 5 is in this sequence because 3*10^5 + 73 = 300073 is prime.
%e A271640 Initial terms and associated primes:
%e A271640 a(1) = 1, 103;
%e A271640 a(2) = 2, 373;
%e A271640 a(3) = 5, 300073;
%e A271640 a(4) = 6, 3000073;
%e A271640 a(5) = 13, 30000000000073, etc.
%t A271640 Select[Range[0, 100000], PrimeQ[3*10^# + 73] &]
%o A271640 (PARI) is(n)=ispseudoprime(3*10^n + 73) \\ _Charles R Greathouse IV_, Jun 13 2017
%Y A271640 Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K A271640 nonn,more
%O A271640 1,2
%A A271640 _Robert Price_, Apr 11 2016
%E A271640 a(34)-a(36) from _Robert Price_, Aug 10 2018