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A293189 Numbers k such that (76*10^k - 211)/9 is prime. 0
1, 2, 4, 5, 8, 13, 41, 55, 82, 188, 296, 320, 464, 509, 1036, 1169, 1808, 2141, 3842, 12274, 23141, 76547, 83816, 85847, 154013 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

For k>1, numbers such that the digit 8 followed by k-2 occurrences of the digit 4 followed by the digits 21 is prime (see Example section).

a(26) > 2*10^5.

LINKS

Table of n, a(n) for n=1..25.

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

Makoto Kamada, Search for 84w21.

EXAMPLE

4 is in this sequence because (76*10^4 - 211)/9 = 84421 is prime.

Initial terms and primes associated:

a(1) = 1, 61;

a(2) = 2, 821;

a(3) = 4, 84421;

a(4) = 5, 844421;

a(5) = 8, 844444421; etc.

MATHEMATICA

Select[Range[1, 100000], PrimeQ[(76*10^# - 211)/9] &]

CROSSREFS

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

Sequence in context: A327045 A288668 A264800 * A278695 A105134 A226791

Adjacent sequences:  A293186 A293187 A293188 * A293190 A293191 A293192

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Oct 01 2017

EXTENSIONS

a(25) from Robert Price, Aug 31 2019

STATUS

approved

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Last modified May 14 22:40 EDT 2021. Contains 343909 sequences. (Running on oeis4.)