A294484 Numbers k such that 7*10^k + 69 is prime.

1, 2, 3, 6, 7, 8, 10, 12, 13, 21, 46, 68, 91, 153, 366, 1112, 1185, 1222, 7161, 20077, 33435, 40747, 99133, 108300, 136716

Offset

1,2

Comments

For k > 1, numbers k such that the digit 7 followed by k-2 occurrences of the digit 0 followed by the digits 69 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 70w69.

Example

2 is in this sequence because 7*10^2 + 69 = 769 is prime.
Initial terms and associated primes:
a(1) = 1, 139;
a(2) = 2, 769;
a(3) = 3, 7069;
a(4) = 6, 7000069;
a(5) = 7, 70000069; etc.

Mathematica

Select[Range[0, 100000], PrimeQ[7*10^# + 69] &]

Crossrefs

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

Sequence in context: A296350 A076436 A028768 * A064528 A366140 A179794

Adjacent sequences: A294481 A294482 A294483 * A294485 A294486 A294487

Keyword

nonn,more,hard

Author

Robert Price, Oct 31 2017

Extensions

a(24)-a(25) from Robert Price, Jul 09 2019

Status

approved