A295970 Numbers k such that (5*10^k + 211)/9 is prime.

1, 2, 5, 8, 32, 37, 38, 53, 104, 128, 241, 866, 872, 943, 1058, 4351, 16471, 16925, 23933, 43511, 73079, 105811

Offset

1,2

Comments

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

a(23) > 2*10^5.

Links

Table of n, a(n) for n=1..22.

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

Makoto Kamada, Search for 5w79.

Example

2 is in this sequence because (5*10^2 + 211)/9 = 79 is prime.
Initial terms and associated primes:
a(1) = 1, 29;
a(2) = 2, 79;
a(3) = 5, 55579;
a(4) = 8, 55555579;
a(5) = 32, 55555555555555555555555555555579; etc.

Mathematica

Select[Range[0, 100000], PrimeQ[(5*10^# + 211)/9] &]

Prog

(PARI) isok(k) = isprime((5*10^k + 211)/9); \\ Michel Marcus, Dec 01 2017

Crossrefs

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

Sequence in context: A290594 A140275 A025533 * A086809 A153863 A153890

Adjacent sequences: A295967 A295968 A295969 * A295971 A295972 A295973

Keyword

nonn,more,hard

Author

Robert Price, Nov 30 2017

Extensions

a(22) from Robert Price, Feb 14 2018

Status

approved