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A290578 Numbers k such that (379*10^k - 1)/9 is prime. 0
1, 2, 5, 8, 10, 11, 20, 56, 161, 172, 263, 290, 578, 800, 1166, 3382, 3848, 7036, 10487, 12101, 36211, 94337 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

For k>0, numbers such that the digits 42 followed by k occurrences of the digit 1 is prime (see Example section).

a(23) > 10^5.

LINKS

Table of n, a(n) for n=1..22.

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

Makoto Kamada, Search for 421w.

EXAMPLE

2 is in this sequence because (379*10^2 - 1)/9 = 4211 is prime.

Initial terms and primes associated:

a(1) = 1, 421;

a(2) = 2, 4211;

a(3) = 5, 4211111;

a(4) = 8; 4211111111;

a(5) = 10, 421111111111; etc.

MATHEMATICA

Select[Range[0, 100000], PrimeQ[(379*10^# - 1)/9] &]

PROG

(PARI) isok(k) = isprime((379*10^k - 1)/9); \\ Michel Marcus, Aug 07 2017

CROSSREFS

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

Sequence in context: A080228 A153052 A237525 * A294127 A166955 A286808

Adjacent sequences:  A290575 A290576 A290577 * A290579 A290580 A290581

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Aug 06 2017

STATUS

approved

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Last modified October 23 12:54 EDT 2019. Contains 328345 sequences. (Running on oeis4.)