A294571 Numbers k such that (71*10^k - 539)/9 is prime.

1, 3, 7, 9, 10, 13, 76, 93, 141, 202, 274, 445, 907, 1233, 1366, 2131, 2590, 2979, 3489, 3735, 95277, 148306, 165663

Offset

1,2

Comments

For k > 1, numbers k such that the digit 7 followed by k-2 occurrences of the digit 8 followed by the digits 29 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 78w29.

Example

3 is in this sequence because (71*10^3 - 539)/9 = 7829 is prime.
Initial terms and associated primes:
a(1) = 1, 19;
a(2) = 3, 7829;
a(3) = 7, 78888829;
a(4) = 9, 7888888829;
a(5) = 10, 78888888829; etc.

Mathematica

Select[Range[1, 100000], PrimeQ[(71*10^#- 539)/9] &]

Crossrefs

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

Sequence in context: A344413 A339618 A357707 * A083214 A091210 A377757

Adjacent sequences: A294568 A294569 A294570 * A294572 A294573 A294574

Keyword

nonn,more,hard

Author

Robert Price, Nov 02 2017

Extensions

a(22)-a(23) from Robert Price, Jul 27 2019

Status

approved