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A259132 Numbers k such that 5*R_k + 7*10^k + 2 is prime, where R_k = 11...11 is the repunit (A002275) of length k. 0
2, 4, 10, 20, 22, 58, 74, 82, 208, 350, 422, 3812, 3982, 20924, 23786, 38852, 56042, 68504, 74434 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Also, numbers k such that (68*10^n+13)/9 is prime.

Terms from Kamada data.

a(20) > 10^5.

LINKS

Table of n, a(n) for n=1..19.

Makoto Kamada, Near-repdigit numbers of the form ABB...BBA.

Makoto Kamada, Prime numbers of the form 755...557.

Index entries for primes involving repunits.

EXAMPLE

For k=2, 5*R_2 + 7*10^k + 2 = 55 + 700 + 2 = 757 which is prime.

MATHEMATICA

Select[Range[0, 100000], PrimeQ[(68*10^#+13)/9] &]

CROSSREFS

Cf. A002275.

Sequence in context: A011963 A083844 A026554 * A099413 A271624 A283090

Adjacent sequences:  A259129 A259130 A259131 * A259133 A259134 A259135

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Jun 18 2015

STATUS

approved

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Last modified January 17 16:47 EST 2018. Contains 297822 sequences.