A102938 - OEIS

A102938

Numbers k such that 10^k + 5*R_k + 4 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

1, 3, 4, 7, 12, 51, 87, 123, 198, 202, 537, 831, 1323, 2508, 2631, 5568, 6897, 7524, 9939, 10674, 11617, 13257, 14371, 20176, 36555, 36874, 40131, 43125, 58729, 97044, 195631

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OFFSET

1,2

COMMENTS

Also numbers k such that (14*10^k + 31)/9 is prime.

a(31) > 10^5. - Robert Price, Feb 03 2015

LINKS

Table of n, a(n) for n=1..31.

Makoto Kamada, Prime numbers of the form 155...559.

Index entries for primes involving repunits.

FORMULA

a(n) = A102021(n) + 1.

MATHEMATICA

Do[ If[ PrimeQ[(14*10^n + 31)/9], Print[n]], {n, 0, 10000}]

CROSSREFS

Cf. A002275, A102021.

Sequence in context: A231337 A373805 A049930 * A076784 A279815 A088764

Adjacent sequences: A102935 A102936 A102937 * A102939 A102940 A102941

KEYWORD

more,nonn

AUTHOR

Robert G. Wilson v, Dec 16 2004

EXTENSIONS

Addition of a(20)-a(24) from Kamada data by Robert Price, Dec 08 2010

a(25)-a(28) from Erik Branger May 01 2013 by Ray Chandler, Aug 16 2013

a(29)-a(30) from Robert Price, Feb 03 2015

a(31) from Kamada data by Tyler Busby, Apr 16 2024

STATUS

approved