

A056698


Numbers k such that 10^k + 3*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.


4



1, 15, 41, 83, 95, 341, 551, 669, 989, 1223, 6923, 103703
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OFFSET

1,2


COMMENTS

Also numbers k such that (4*10^k1)/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^n1)/9], Print[n]], {n, 0, 30470}]


PROG

(PARI) for(k=1, 1500, if(ispseudoprime(4*(10^k1)/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



