A294941 Numbers k such that (14*10^k + 211)/9 is prime.

2, 3, 5, 6, 8, 9, 18, 41, 209, 815, 2573, 3590, 4439, 4568, 6720, 16491, 20069, 22406, 36756, 43076, 53465, 58550, 70896, 74858, 184950

Offset

1,1

Comments

For k > 1, numbers k such that the digit 1 followed by k-2 occurrences of the digit 5 followed by the digits 79 is prime (see Example section).

a(26) > 2*10^5.

Links

Table of n, a(n) for n=1..25.

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

Makoto Kamada, Search for 15w79.

Example

2 is in this sequence because (14*10^2 + 211)/9 = 179 s prime.
Initial terms and associated primes:
a(1) = 2, 179;
a(2) = 3, 1579;
a(3) = 5, 155579;
a(4) = 6, 1555579;
a(5) = 8, 155555579; etc.

Mathematica

Select[Range[0 100000], PrimeQ[(14*10^# + 211)/9] &]

Prog

(PARI) isok(k) = isprime((14*10^k + 211)/9); \\ Michel Marcus, Nov 12 2017

Crossrefs

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

Sequence in context: A299101 A335658 A364807 * A028374 A050578 A283513

Adjacent sequences: A294938 A294939 A294940 * A294942 A294943 A294944

Keyword

nonn,more,hard

Author

Robert Price, Nov 11 2017

Extensions

a(25) from Robert Price, Apr 21 2018

Status

approved