A274976 - OEIS

%I #28 May 02 2024 04:27:59

%S 0,1,2,3,4,7,9,57,98,122,123,249,304,318,339,374,390,476,619,1358,

%T 1724,3351,5046,5572,6685,9421,14362,97353

%N Numbers k such that (26*10^k + 31)/3 is prime.

%C For k > 1, numbers k such that the digit 8 followed by k-2 occurrences of the digit 6 followed by the digits 77 is prime (see Example section).

%C a(29) > 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 86w77</a>.

%e 3 is in this sequence because (26*10^3 + 31)/3 = 877 is prime.

%e Initial terms and associated primes:

%e a(1) = 0, 19;

%e a(2) = 1, 97;

%e a(3) = 2, 877;

%e a(4) = 3, 8677;

%e a(5) = 4, 86677, etc.

%t Select[Range[0, 100000], PrimeQ[(26*10^# + 31)/3] &]

%o (PARI) is(n)=ispseudoprime((26*10^n+31)/3) \\ _Charles R Greathouse IV_, Jun 13 2017

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

%K nonn,more

%O 1,3

%A _Robert Price_, Jul 14 2016