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A293842 Numbers k such that 3*10^k - 31 is prime. 0

%I

%S 2,3,5,10,21,101,133,187,209,363,747,1550,3715,3875,6022,9729,10460,

%T 12316,14421,19023,50727

%N Numbers k such that 3*10^k - 31 is prime.

%C For k>1, numbers such that the digit 2 followed by k-2 occurrences of the digit 9 followed by the digits 69 is prime (see Example section).

%C a(22) > 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/primedifficulty.txt">Search for 29w69</a>

%e 3 is in this sequence because 3*10^3 - 31 = 2969 is prime.

%e Initial terms and primes associated:

%e a(1) = 2, 269;

%e a(2) = 3, 2969;

%e a(3) = 5, 299969;

%e a(4) = 10, 29999999969;

%e a(5) = 21, 2999999999999999999969; etc.

%t Select[Range[2, 100000], PrimeQ[3*10^# - 31] &]

%Y Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.

%K nonn,more,hard

%O 1,1

%A _Robert Price_, Oct 18 2017

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Last modified July 4 11:31 EDT 2020. Contains 335448 sequences. (Running on oeis4.)