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 A056714 Numbers k such that 5*10^k + 3*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k. 1
 0, 1, 3, 13, 25, 49, 143, 419, 1705, 13993, 35753, 40889 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS Also numbers k such that (16*10^k - 1)/3 is prime. 5*10^a(n) + 3*(10^a(n) - 1)/9 is a solution for part (b) of questions of puzzle 244 from www.primepuzzles.net. If a(n) is greater than 5812 then a(n) is an example that is asked for in this question. - Farideh Firoozbakht, Dec 02 2003 LINKS Makoto Kamada, Prime numbers of the form 533...33. MATHEMATICA Do[ If[ PrimeQ[ 5*10^n + 3*(10^n-1)/9], Print[n]], {n, 0, 5000}] CROSSREFS Cf. A002275, A093674. Sequence in context: A030552 A146371 A146379 * A120074 A056706 A052454 Adjacent sequences:  A056711 A056712 A056713 * A056715 A056716 A056717 KEYWORD nonn AUTHOR Robert G. Wilson v, Aug 11 2000 EXTENSIONS 1705 from Farideh Firoozbakht, Dec 18 2003 13993, 35753 and 40889 from Serge Batalov, Jan 2009 confirmed as next terms by Ray Chandler, Feb 11 2012 STATUS approved

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Last modified December 5 08:36 EST 2021. Contains 349543 sequences. (Running on oeis4.)