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 A098930 Numbers k such that 2*10^k + 5*R_k - 4 is prime, where R_k = 11...1 is the repunit (A002275) of length k. 1
 2, 3, 5, 6, 12, 186, 435, 1746, 3447, 18798, 70209 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Also numbers k such that (23*10^k - 41)/9 is prime. a(12) > 10^5. - Robert Price, Mar 04 2015 LINKS Makoto Kamada, Prime numbers of the form 255...551. FORMULA a(n) = A101961(n) + 1. EXAMPLE If n = 3 we get 2551, which is prime. MATHEMATICA Do[ If[ PrimeQ[(23*10^n - 41)/9], Print[n]], {n, 0, 10000}] CROSSREFS Cf. A002275, A101961. Sequence in context: A125877 A118787 A191783 * A075372 A064725 A301761 Adjacent sequences:  A098927 A098928 A098929 * A098931 A098932 A098933 KEYWORD more,nonn AUTHOR Julien Peter Benney (jpbenney(AT)ftml.net), Oct 20 2004 EXTENSIONS a(7) from Ray Chandler, Nov 04 2004 a(8) & a(9) from Robert G. Wilson v, Dec 17 2004 Addition of a(10) from Kamada data by Robert Price, Dec 13 2010 a(11) from Robert Price, Mar 04 2015 STATUS approved

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Last modified November 30 17:12 EST 2021. Contains 349424 sequences. (Running on oeis4.)