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 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
 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 (list; graph; refs; listen; history; text; internal format)
 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 Makoto Kamada, Prime numbers of the form 155...559. 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: A124636 A231337 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 STATUS approved

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Last modified December 4 18:08 EST 2021. Contains 349526 sequences. (Running on oeis4.)