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A281643 Numbers k such that 7*10^k - 69 is prime. 0
2, 4, 5, 12, 29, 34, 53, 56, 161, 287, 312, 436, 468, 672, 1316, 1642, 2814, 4171, 6409, 6657, 7914, 9259, 9997, 21253, 29180, 37246, 47936, 48816, 49580, 92414, 97874, 105131, 119006, 137153, 151309, 155450 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

For k>1, numbers such that the digit 6 followed by k-2 occurrences of the digit 9 followed by the digits 31 is prime (see Example section).

a(37) > 2*10^5.

LINKS

Table of n, a(n) for n=1..36.

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

Makoto Kamada, Search for 69w31.

EXAMPLE

4 is in this sequence because 7*10^4 - 69 = 69931 is prime.

Initial terms and primes associated:

a(1) = 2, 631;

a(2) = 4, 69931;

a(3) = 5, 699931;

a(4) = 12, 6999999999931;

a(5) = 29, 699999999999999999999999999931; etc.

MATHEMATICA

Select[Range[1, 100000], PrimeQ[7*10^# - 69] &]

CROSSREFS

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

Sequence in context: A117556 A091071 A050599 * A102932 A128457 A139485

Adjacent sequences:  A281640 A281641 A281642 * A281644 A281645 A281646

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Jan 25 2017

EXTENSIONS

a(32)-a(36) from Robert Price, Jul 03 2019

STATUS

approved

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Last modified October 24 20:35 EDT 2021. Contains 348233 sequences. (Running on oeis4.)