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A290330 Numbers k such that 8*10^k - 51 is prime. 0
1, 3, 5, 28, 90, 209, 220, 226, 301, 352, 516, 1746, 2214, 2215, 2364, 2413, 3043, 3895, 5837, 8283, 16551, 20955, 46161, 67302, 82360, 94357, 100439 (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 9 followed by the digits 49 is prime (see Example section).

a(28) > 2*10^5.

LINKS

Table of n, a(n) for n=1..27.

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

Makoto Kamada, Search for 79w49.

EXAMPLE

3 is in this sequence because 8*10^3 - 51 = 7949 is prime.

Initial terms and primes associated:

a(1) = 1, 29;

a(2) = 3, 7949;

a(3) = 5, 799949;

a(4) = 28, 79999999999999999999999999949; etc.

MATHEMATICA

Select[Range[1, 100000], PrimeQ[8*10^# - 51] &]

PROG

(PARI) is(n) = ispseudoprime(8*10^n-51) \\ Felix Fröhlich, Aug 12 2019

CROSSREFS

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

Sequence in context: A103526 A054388 A110255 * A300676 A016552 A321069

Adjacent sequences:  A290327 A290328 A290329 * A290331 A290332 A290333

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Oct 06 2017

EXTENSIONS

a(27) from Robert Price, Aug 12 2019

STATUS

approved

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Last modified September 23 20:42 EDT 2021. Contains 347617 sequences. (Running on oeis4.)