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A056698 Numbers n such that 10^n + 3*R_n is prime, where R_n = 11...1 is the repunit (A002275) of length n. 4
1, 15, 41, 83, 95, 341, 551, 669, 989, 1223, 6923, 103703 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Also numbers n such that (4*10^n-1)/3 is prime.

a(13) > 850000 (from Kamada data). - Robert Price, Oct 19 2014

LINKS

Table of n, a(n) for n=1..12.

Makoto Kamada, Prime numbers of the form 133...33.

Index entries for primes involving repunits

MATHEMATICA

Do[ If[ PrimeQ[ 10^n + 3*(10^n-1)/9], Print[n]], {n, 0, 30470}]

PROG

(PARI) for(k=1, 1500, if(ispseudoprime(4*(10^k-1)/3+1), print1(k, ", "))) \\ Hugo Pfoertner, Jul 22 2020

CROSSREFS

Cf. A002275, A093671, A097166, A259050.

Sequence in context: A103003 A239325 A024845 * A070007 A325041 A154267

Adjacent sequences:  A056695 A056696 A056697 * A056699 A056700 A056701

KEYWORD

nonn,hard,more

AUTHOR

Robert G. Wilson v, Aug 10 2000

EXTENSIONS

a(12) from Kamada data by Robert Price, Oct 19 2014

STATUS

approved

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Last modified October 27 08:45 EDT 2020. Contains 338035 sequences. (Running on oeis4.)