A291661 Numbers k such that (58*10^k + 221)/9 is prime.

0, 1, 3, 7, 13, 18, 19, 36, 78, 129, 171, 175, 237, 607, 619, 717, 954, 1386, 3948, 7939, 16519, 17686, 18759, 19986, 36340, 39547, 44014, 60579, 170842

Offset

1,3

Comments

For k>1, numbers such that the digit 6 followed by k-2 occurrences of the digit 4 followed by the digits 69 is prime (see Example section).

a(30) > 5*10^5.

Links

Table of n, a(n) for n=1..29.

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

Makoto Kamada, Search for 64w69

Example

3 is in this sequence because (58*10^3 + 221)/9 = 6469 is prime.
Initial terms and primes associated:
a(1) = 0, 31;
a(2) = 1, 89;
a(3) = 3, 6469;
a(4) = 7, 64444469;
a(5) = 13, 64444444444469; etc.

Mathematica

Select[Range[0, 100000], PrimeQ[(58*10^# + 221)/9] &]

Crossrefs

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

Sequence in context: A076746 A243706 A214696 * A310256 A310257 A310258

Adjacent sequences: A291658 A291659 A291660 * A291662 A291663 A291664

Keyword

nonn,more,hard

Author

Robert Price, Aug 28 2017

Status

approved