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A282457 Numbers k such that 7*10^k - 47 is prime. 0
1, 2, 5, 8, 11, 14, 15, 30, 31, 42, 69, 85, 120, 128, 449, 685, 686, 741, 2165, 9110, 9150, 11366, 13154, 1291, 1521, 33321, 40758, 50789, 51440, 65492, 82509, 93887 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

For k>1, numbers such that the digit 6 followed by k-2 occurrences of the digit 9 followed by the digits 53 is prime (see Example section).

a(33) > 2*10^5.

LINKS

Table of n, a(n) for n=1..32.

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

Makoto Kamada, Search for 69w53.

EXAMPLE

2 is in this sequence because 7*10^2 - 47 = 653 is prime.

Initial terms and primes associated:

a(1) = 1, 23;

a(2) = 2, 653;

a(3) = 5, 699953;

a(4) = 8, 699999953;

a(5) = 11, 699999999953; etc.

MATHEMATICA

Select[Range[1, 100000], PrimeQ[7*10^# - 47] &]

CROSSREFS

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

Sequence in context: A294391 A257771 A276881 * A329848 A059560 A022842

Adjacent sequences:  A282454 A282455 A282456 * A282458 A282459 A282460

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Feb 15 2017

STATUS

approved

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Last modified February 27 06:36 EST 2020. Contains 332299 sequences. (Running on oeis4.)