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A256458 Numbers k such that 3*R_(k+2) + 10^k is prime, where R_k = 11...1 is the repunit (A002275) of length n. 0
2, 4, 5, 6, 11, 26, 92, 98, 110, 215, 232, 354, 522, 648, 1862, 2322, 2934, 3797, 6017, 11236, 24748, 30011, 37864, 42904, 57414, 195478 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Also, numbers k such that (103*10^n-1)/3  is prime.

Terms from Kamada.

a(27) > 250000.

LINKS

Table of n, a(n) for n=1..26.

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

Makoto Kamada, Prime numbers of the form 3433...33.

Index entries for primes involving repunits.

EXAMPLE

For k=4, 3*R_6 + 10^4 = 333333 + 10000 = 343333 which is prime.

MATHEMATICA

Select[Range[0, 250000], PrimeQ[(103*10^#-1)/3 ] &]

CROSSREFS

Cf. A002275.

Sequence in context: A194637 A194600 A255543 * A276001 A182109 A006539

Adjacent sequences:  A256455 A256456 A256457 * A256459 A256460 A256461

KEYWORD

more,hard,nonn

AUTHOR

Robert Price, Apr 14 2015

STATUS

approved

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Last modified July 6 23:01 EDT 2020. Contains 335484 sequences. (Running on oeis4.)