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A256788 Numbers k such that 3*R_(k+2) + 2*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k. 0
1, 2, 4, 16, 23, 25, 27, 29, 34, 68, 89, 127, 200, 418, 868, 1173, 2553, 4131, 5675, 8318, 8445, 10820, 15793, 16671, 32275, 43995, 48026, 50035, 88459, 126839 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Also, numbers k such that (106*10^k - 1)/3  is prime.

Terms from Kamada.

a(31) > 250000.

LINKS

Table of n, a(n) for n=1..30.

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

Makoto Kamada, Prime numbers of the form 3533...33.

Index entries for primes involving repunits.

EXAMPLE

For k=4, 3*R_6 + 2*10^4 = 333333 + 20000 = 353333 which is prime.

MATHEMATICA

Select[Range[0, 250000], PrimeQ[(106*10^#-1)/3 ] &]

CROSSREFS

Cf. A002275.

Sequence in context: A049441 A253143 A193868 * A324211 A059622 A131475

Adjacent sequences:  A256785 A256786 A256787 * A256789 A256790 A256791

KEYWORD

more,hard,nonn

AUTHOR

Robert Price, Apr 14 2015

STATUS

approved

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Last modified September 24 15:40 EDT 2021. Contains 347643 sequences. (Running on oeis4.)