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A286936 Numbers k such that (71*10^k - 287)/9 is prime. 0
1, 2, 4, 8, 26, 28, 43, 70, 92, 128, 331, 364, 478, 532, 689, 778, 895, 1210, 5081, 7855, 17852, 20864, 42598, 56858 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

For k>1, numbers such that the digit 7 followed by k-2 occurrences of the digit 8 followed by the digits 57 is prime (see Example section).

a(25) > 10^5.

LINKS

Table of n, a(n) for n=1..24.

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

Makoto Kamada, Search for 78w57.

EXAMPLE

4 is in this sequence because (71*10^4 - 287)/9 = 78857 is prime.

Initial terms and primes associated:

a(1) = 1, 47;

a(2) = 2, 757;

a(3) = 4, 78857;

a(4) = 8, 788888857;

a(5) = 26, 788888888888888888888888857; etc.

MATHEMATICA

Select[Range[1, 100000], PrimeQ[(71*10^# - 287)/9] &]

CROSSREFS

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

Sequence in context: A191700 A000643 A261489 * A323948 A196265 A112285

Adjacent sequences:  A286933 A286934 A286935 * A286937 A286938 A286939

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, May 16 2017

STATUS

approved

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Last modified May 21 15:32 EDT 2019. Contains 323444 sequences. (Running on oeis4.)