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A257030 Numbers k such that 7*R_(k+2) - 3*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length n. 0
2, 5, 23, 182, 209, 287, 7454, 16958 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Also, numbers k such that (673*10^n-7)/9 is prime.

Terms from Kamada.

a(9) > 30000.

LINKS

Table of n, a(n) for n=1..8.

Makoto Kamada, Near-repdigit numbers of the form ABAA...AA.

Makoto Kamada, Prime numbers of the form 7477...77.

Index entries for primes involving repunits.

EXAMPLE

For k=5, 7*R_7 - 3*10^5 = 7777777 - 300000 = 7477777 which is prime.

MATHEMATICA

Select[Range[0, 30000], PrimeQ[(673*10^#-7)/9 ] &]

PROG

(MAGMA) [n: n in [0..400] | IsPrime((673*10^n-7) div 9)]; // Vincenzo Librandi, Apr 15 2015

CROSSREFS

Cf. A002275.

Sequence in context: A343020 A167142 A136731 * A062495 A158889 A181074

Adjacent sequences:  A257027 A257028 A257029 * A257031 A257032 A257033

KEYWORD

more,hard,nonn

AUTHOR

Robert Price, Apr 14 2015

STATUS

approved

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Last modified May 6 09:45 EDT 2021. Contains 343580 sequences. (Running on oeis4.)