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%I #23 May 26 2024 16:04:27
%S 2,4,5,8,9,15,19,25,26,137,141,257,399,508,524,607,709,953,989,2484,
%T 3196,4538,6448,35401,36106,51530,79327,109316,171891,184004
%N Numbers k such that 4*10^k - 63 is prime.
%C For k > 1, numbers k such that the digit 3 followed by k-2 occurrences of the digit 9 followed by the digits 37 is prime (see Example section).
%C a(31) > 3*10^5.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr">Factorization of near-repdigit-related numbers</a>.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/prime/prime_difficulty.txt">Search for 39w37</a>.
%e 4 is in this sequence because 4*10^4 - 63 = 39937 is prime.
%e Initial terms and associated primes:
%e a(1) = 2, 337;
%e a(2) = 4, 39937;
%e a(3) = 5, 399937;
%e a(4) = 8, 399999937;
%e a(5) = 9, 3999999937; etc.
%t Select[Range[2, 100000], PrimeQ[4*10^# - 63] &]
%o (PARI) is(n)=ispseudoprime(4*10^n - 63) \\ _Charles R Greathouse IV_, Jun 13 2017
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more,hard
%O 1,1
%A _Robert Price_, Dec 18 2016
%E a(28)-a(30) from _Robert Price_, Oct 24 2018
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