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A295394 Numbers k such that (2*10^k - 179)/3 is prime. 0
2, 3, 4, 6, 9, 25, 162, 258, 457, 813, 1079, 1266, 2397, 4075, 15688, 18663, 21079, 21851, 40627, 54960 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

For k > 1, numbers such that k-2 occurrences of the digit 6 followed by the digits 07 is prime (see Example section).

a(21) > 2*10^5.

LINKS

Table of n, a(n) for n=1..20.

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

Makoto Kamada, Search for 6w07

EXAMPLE

3 is in this sequence because (2*10^3 - 179)/3 = 607 is prime.

Initial terms and primes associated:

a(1) = 2, 7;

a(2) = 3, 607;

a(3) = 4, 6607;

a(4) = 6, 666607;

a(5) = 9, 666666607; etc.

MATHEMATICA

Select[Range[2, 100000], PrimeQ[(2*10^# - 179)/3] &]

PROG

(PARI) isok(k) = isprime((2*10^k - 179)/3); \\ Michel Marcus, Nov 22 2017

CROSSREFS

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

Sequence in context: A103481 A215285 A275173 * A081419 A108858 A294688

Adjacent sequences:  A295391 A295392 A295393 * A295395 A295396 A295397

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Nov 21 2017

STATUS

approved

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Last modified October 15 15:03 EDT 2019. Contains 328030 sequences. (Running on oeis4.)