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A295323 Numbers k such that 2*10^k - 51 is prime. 0
2, 3, 4, 8, 11, 13, 17, 28, 56, 105, 231, 339, 643, 922, 1219, 1880, 2209, 4238, 4987, 14770, 56194, 67043, 96867, 124751 (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 9 followed by the digits 49 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 19w49

EXAMPLE

2 is in this sequence because 2*10^2 - 51 = 149 is prime.

Initial terms and primes associated:

a(1) = 2, 149;

a(2) = 3, 1949;

a(3) = 4, 19949;

a(4) = 8, 199999949;

a(5) = 11, 199999999949; etc.

MATHEMATICA

Select[Range[2, 100000], PrimeQ[2*10^# - 51] &]

PROG

(PARI) isok(k) = isprime(2*10^k - 51); \\ Michel Marcus, Nov 20 2017

CROSSREFS

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

Sequence in context: A275646 A279194 A050727 * A102951 A015935 A135909

Adjacent sequences:  A295320 A295321 A295322 * A295324 A295325 A295326

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Nov 19 2017

EXTENSIONS

a(24) from Robert Price, Apr 13 2018

STATUS

approved

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Last modified September 29 21:59 EDT 2020. Contains 337432 sequences. (Running on oeis4.)