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A273924 Numbers k such that (7*10^k - 13)/3 is prime.

%I A273924 #18 May 02 2024 04:28:06
%S A273924 1,2,5,6,28,53,56,86,88,90,96,136,142,186,202,373,448,785,988,1263,
%T A273924 1966,3561,4768,9658,9831,17797,42286,49893,98007,129472,146860
%N A273924 Numbers k such that (7*10^k - 13)/3 is prime.
%C A273924 For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 3 followed by the digits 29 is prime (see Example section).
%C A273924 a(32) > 2*10^5.
%H A273924 Makoto Kamada, <a href="https://stdkmd.net/nrr">Factorization of near-repdigit-related numbers</a>.
%H A273924 Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/prime_difficulty.txt">Search for 23w29</a>.
%e A273924 5 is in this sequence because (7*10^5 - 13)/3 = 233329 is prime.
%e A273924 Initial terms and associated primes:
%e A273924 a(1) = 1, 19;
%e A273924 a(2) = 2, 229;
%e A273924 a(3) = 5, 233329;
%e A273924 a(4) = 6, 2333329;
%e A273924 a(5) = 28, 23333333333333333333333333329, etc.
%t A273924 Select[Range[0, 100000], PrimeQ[(7*10^# - 13)/3] &]
%o A273924 (PARI) is(n)=ispseudoprime((7*10^n - 13)/3) \\ _Charles R Greathouse IV_, Jun 08 2016
%Y A273924 Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K A273924 nonn,more
%O A273924 1,2
%A A273924 _Robert Price_, Jun 04 2016
%E A273924 a(30)-a(31) from _Robert Price_, Jul 13 2018