A294940 Numbers k such that (14*10^k + 229)/9 is prime.

1, 2, 4, 5, 8, 10, 145, 319, 620, 629, 1165, 5897, 8797, 10138, 10897, 13438, 21863, 23089, 27400, 51079, 182000

Offset

1,2

Comments

For k > 1, numbers such that the digit 1 followed by k-2 occurrences of the digit 5 followed by the digits 81 is prime (see Example section).

a(22) > 2*10^5.

Links

Table of n, a(n) for n=1..21.

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

Makoto Kamada, Search for 15w81

Example

2 is in this sequence because (59*10^2 - 77)/9 = 181 is prime.
Initial terms and primes associated:
a(1) = 1, 41;
a(2) = 2, 181;
a(3) = 4, 15581;
a(4) = 5, 155581;
a(5) = 8, 155555581; etc.

Mathematica

Select[Range[0, 100000], PrimeQ[(59*10^# - 77)/9] &]

Prog

(PARI) isok(k) = isprime((14*10^k + 229)/9); \\ Michel Marcus, Nov 12 2017

Crossrefs

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

Sequence in context: A018332 A018257 A103075 * A322125 A174803 A282434

Adjacent sequences: A294937 A294938 A294939 * A294941 A294942 A294943

Keyword

nonn,more,hard

Author

Robert Price, Nov 11 2017

Extensions

a(21) from Robert Price, Mar 14 2018

Status

approved