Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #16 May 02 2024 04:27:02
%S 2,5,6,10,16,29,35,82,107,170,185,204,223,226,388,512,1586,2137,3182,
%T 7325,7346,8143,8746,11322,11497,13279,44681,108624,183872
%N Numbers k such that 3*10^k - 49 is prime.
%C For k > 1, numbers k such that the digit 2 followed by k-2 occurrences of the digit 9 followed by the digits 51 is prime (see Example section).
%C a(31) > 2*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 29w51</a>.
%e 5 is in this sequence because 3*10^5-49 = 299951 is prime.
%e Initial terms and associated primes:
%e a(1) = 2, 251;
%e a(2) = 5, 299951;
%e a(3) = 6, 2999951;
%e a(4) = 10, 29999999951;
%e a(5) = 16, 29999999999999951, etc.
%t Select[Range[0, 100000], PrimeQ[3*10^# - 49] &]
%o (PARI) is(n)=ispseudoprime(3*10^n - 49) \\ _Charles R Greathouse IV_, Jun 08 2016
%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
%K nonn,more
%O 1,1
%A _Robert Price_, Jun 07 2016
%E a(28)-a(29) from _Robert Price_, Jul 29 2018