A294939 Numbers k such that (8*10^k + 13)/3 is prime.

0, 1, 2, 3, 5, 8, 13, 69, 80, 344, 405, 500, 794, 926, 3293, 3964, 7203, 7395, 8163, 14433, 68455, 108273, 137845

Offset

1,3

Comments

For k > 1, numbers such that the digit 2 followed by k-2 occurrences of the digit 6 followed by the digits 71 is prime (see Example section).

a(24) > 2*10^5.

Links

Table of n, a(n) for n=1..23.

Makoto Kamada, Factorization of near-repdigit-related numbers.

Makoto Kamada, Search for 26w71

Example

2 is in this sequence because (8*10^2 + 13)/3 = 271 is prime.
Initial terms and primes associated:
a(1) = 0, 7;
a(2) = 1, 31;
a(3) = 2, 271;
a(4) = 3, 2671;
a(5) = 5, 266671; etc.

Mathematica

Select[Range[0, 100000], PrimeQ[(8*10^# + 13)/3] &]

Prog

(PARI) isok(k) = isprime((8*10^k + 13)/3); \\ Michel Marcus, Nov 12 2017

Crossrefs

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

Sequence in context: A281599 A041957 A329192 * A096095 A041101 A041809

Adjacent sequences: A294936 A294937 A294938 * A294940 A294941 A294942

Keyword

nonn,more,hard

Author

Robert Price, Nov 11 2017

Extensions

a(22)-a(23) from Robert Price, Jun 12 2018

Status

approved