A102931 - OEIS

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A102931

Numbers k such that 10^k + 2*R_k + 7 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

2

1, 3, 6, 51, 57, 63, 150, 393, 420, 547, 42024, 43063, 101613, 107385

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OFFSET

1,2

COMMENTS

Also numbers k such that (11*10^k + 61)/9 is prime.

a(13) > 10^5. - Robert Price, Jan 17 2015

a(15) > 2*10^5. - Tyler Busby, Feb 01 2023

LINKS

Table of n, a(n) for n=1..14.

Makoto Kamada, Prime numbers of the form 122...229.

Index entries for primes involving repunits.

FORMULA

a(n) = A102011(n) + 1.

MATHEMATICA

Do[ If[ PrimeQ[(11*10^n + 61)/9], Print[n]], {n, 0, 10000}]

CROSSREFS

Cf. A002275, A102011.

Sequence in context: A032322 A203765 A111752 * A056447 A056437 A309320

Adjacent sequences: A102928 A102929 A102930 * A102932 A102933 A102934

KEYWORD

more,nonn

AUTHOR

Robert G. Wilson v, Dec 16 2004

EXTENSIONS

a(11)-a(12) from Erik Branger May 01 2013 by Ray Chandler, Aug 16 2013

a(13)-a(14) from Tyler Busby, Jan 31 2023

STATUS

approved