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A295324 Numbers k such that (17*10^k - 89)/9 is prime. 0
2, 3, 6, 12, 39, 45, 50, 75, 126, 282, 431, 459, 1557, 2724, 3453, 4058, 15879, 24209, 31844, 61760, 155133 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

For k > 1, numbers such that the digit 1 followed by k-2 occurrences of the digit 8 followed by the digits 79 is prime (see Example section).

a(22) > 2*10^5.

LINKS

Table of n, a(n) for n=1..21.

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

Makoto Kamada, Search for 18w79

EXAMPLE

2 is in this sequence because (17*10^2 - 89)/9 = 179 is prime.

Initial terms and primes associated:

a(1) = 2, 179;

a(2) = 3, 1879;

a(3) = 6, 1888879;

a(4) = 12, 1888888888879;

a(5) = 39, 1888888888888888888888888888888888888879; etc.

MATHEMATICA

Select[Range[1, 100000], PrimeQ[(17*10^# - 89)/9] &]

PROG

(PARI) isok(k) = isprime((17*10^k - 89)/9); \\ Michel Marcus, Nov 20 2017

CROSSREFS

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

Sequence in context: A102970 A144771 A187013 * A054049 A276690 A175281

Adjacent sequences:  A295321 A295322 A295323 * A295325 A295326 A295327

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Nov 19 2017

EXTENSIONS

a(21) from Robert Price, Jul 23 2018

STATUS

approved

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Last modified August 4 19:48 EDT 2020. Contains 336202 sequences. (Running on oeis4.)