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A286434 Numbers k such that (2*10^k - 59)/3 is prime. 0
2, 3, 6, 9, 10, 14, 15, 34, 56, 138, 250, 350, 357, 374, 392, 1594, 4794, 5290, 6702, 11936, 22296, 55762, 55834, 96195 (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 47 is prime (see Example section).

a(25) > 2*10^5.

LINKS

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

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

Makoto Kamada, Search for 6w47.

EXAMPLE

3 is in this sequence because (2*10^3 - 59)/3 = 647 is prime.

Initial terms and primes associated:

a(1) = 2, 47;

a(2) = 3, 647;

a(3) = 6, 666647;

a(4) = 9, 666666647;

a(5) = 10, 6666666647; etc.

MATHEMATICA

Select[Range[2, 100000], PrimeQ[(2*10^# - 59)/3] &]

PROG

(PARI) isok(k) = isprime((2*10^k-59)/3); \\ Michel Marcus, Mar 06 2018

CROSSREFS

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

Sequence in context: A268692 A089160 A214974 * A288930 A045588 A191178

Adjacent sequences:  A286431 A286432 A286433 * A286435 A286436 A286437

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, May 09 2017

STATUS

approved

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Last modified February 25 20:04 EST 2020. Contains 332258 sequences. (Running on oeis4.)