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A271585 Numbers k such that (7*10^k + 143)/3 is prime.

%I A271585 #13 May 25 2024 15:38:17
%S A271585 1,2,3,6,7,10,11,25,26,32,122,123,126,161,292,320,743,1630,2738,3178,
%T A271585 4814,4833,5030,7035,8151,12554,13954,15113,80490,96112,121487,190683
%N A271585 Numbers k such that (7*10^k + 143)/3 is prime.
%C A271585 For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 3 followed by the digits 81 is prime (see Example section).
%C A271585 a(33) > 2*10^5.
%H A271585 Makoto Kamada, <a href="https://stdkmd.net/nrr">Factorization of near-repdigit-related numbers</a>.
%H A271585 Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/prime_difficulty.txt">Search for 23w81</a>.
%e A271585 3 is in this sequence because (7*10^3 + 143)/3 = 2381 is prime.
%e A271585 Initial terms and associated primes:
%e A271585 a(1) = 1, 71;
%e A271585 a(2) = 2, 281;
%e A271585 a(3) = 3, 2381;
%e A271585 a(4) = 6, 2333381;
%e A271585 a(5) = 7, 23333381, etc.
%t A271585 Select[Range[0, 100000], PrimeQ[(7*10^# + 143)/3] &]
%o A271585 (PARI) is(n)=ispseudoprime((7*10^n+143)/3) \\ _Charles R Greathouse IV_, Jun 13 2017
%Y A271585 Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K A271585 nonn,more
%O A271585 1,2
%A A271585 _Robert Price_, Apr 10 2016
%E A271585 a(31)-a(32) from _Robert Price_, Mar 30 2018