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A259135 Numbers k such that 2*R_k + 7*10^k + 5 is prime, where R_k = 11...11 is the repunit (A002275) of length k. 0
2, 4, 8, 28, 64, 724, 1786, 7276, 19462, 24214, 51778 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Also, numbers k such that (65*10^k + 43)/9 is prime.

Terms from Kamada data.

a(12) > 10^5.

LINKS

Table of n, a(n) for n=1..11.

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

Makoto Kamada, Prime numbers of the form 72...27.

Index entries for primes involving repunits.

EXAMPLE

For k=2, 2*R_2 + 7*10^k + 5 = 22 + 700 + 5 = 727 which is prime.

MATHEMATICA

Select[Range[0, 100000], PrimeQ[(65*10^#+43)/9] &]

CROSSREFS

Cf. A002275.

Sequence in context: A324328 A056800 A302915 * A219969 A034919 A298682

Adjacent sequences:  A259132 A259133 A259134 * A259136 A259137 A259138

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Jun 18 2015

STATUS

approved

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Last modified September 29 22:48 EDT 2022. Contains 357092 sequences. (Running on oeis4.)