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A257026 Numbers k such that 3*R_(k+2) + 5*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length n. 0
1, 2, 3, 9, 10, 13, 19, 25, 33, 124, 150, 175, 199, 290, 301, 506, 592, 1016, 1459, 4150, 6396, 7059, 8311, 8355, 8811, 13200, 13270, 26775, 33303, 62073, 66043, 154869, 162153, 172862 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Also, numbers k such that (115*10^n-1)/3 is prime.

Terms from Kamada.

a(35) > 250000.

LINKS

Table of n, a(n) for n=1..34.

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

Makoto Kamada, Prime numbers of the form 3833...33.

Index entries for primes involving repunits.

EXAMPLE

For k=2, 3*R_4 + 5*10^2 = 3333 + 500 = 3833 which is prime.

MATHEMATICA

Select[Range[0, 250000], PrimeQ[(115*10^#-1)/3 ] &]

PROG

(PARI) for(n=0, 300, if(isprime((115*10^n-1)/3), print1(n, ", "))) \\ Derek Orr, Apr 14 2015

(MAGMA) [n: n in [0..300] | IsPrime((115*10^n-1) div 3)]; // Vincenzo Librandi, Apr 15 2015

CROSSREFS

Cf. A002275.

Sequence in context: A039012 A291924 A214433 * A057291 A135107 A191568

Adjacent sequences:  A257023 A257024 A257025 * A257027 A257028 A257029

KEYWORD

more,hard,nonn

AUTHOR

Robert Price, Apr 14 2015

STATUS

approved

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Last modified April 6 10:28 EDT 2020. Contains 333273 sequences. (Running on oeis4.)