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A259130 Numbers k such that 4*R_k + 3*10^k - 1 is prime, where R_k = 11...11 is the repunit (A002275) of length k. 0
0, 6, 12, 492, 5568, 24756 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Also, numbers k such that (31*10^n-13)/9 is prime.

Terms from Kamada data.

Note Kamada does not recognize k=0 as 2 is a degenerate case of form ABB..BBA.

a(7) > 2*10^5.

LINKS

Table of n, a(n) for n=1..6.

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

Makoto Kamada, Prime numbers of the form 344...443.

Index entries for primes involving repunits.

EXAMPLE

For k=6, 4*R_6 + 3*10^k - 1 = 444444 + 3000000 - 1 = 3444443 which is prime.

MATHEMATICA

Select[Range[0, 100000], PrimeQ[(31*10^#-13)/9] &]

CROSSREFS

Cf. A002275.

Sequence in context: A097174 A325030 A191415 * A032511 A036900 A159282

Adjacent sequences:  A259127 A259128 A259129 * A259131 A259132 A259133

KEYWORD

nonn,more,hard

AUTHOR

Robert Price, Jun 18 2015

STATUS

approved

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Last modified June 5 09:20 EDT 2020. Contains 334829 sequences. (Running on oeis4.)