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A056714
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Numbers k such that 5*10^k + 3*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
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1
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0, 1, 3, 13, 25, 49, 143, 419, 1705, 13993, 35753, 40889
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OFFSET
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1,3
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COMMENTS
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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
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LINKS
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Table of n, a(n) for n=1..12.
Makoto Kamada, Prime numbers of the form 533...33.
Carlos Rivera, Puzzle 244. Null Conjunction, The Prime Puzzles and Problems Connection.
Index entries for primes involving repunits
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MATHEMATICA
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Do[ If[ PrimeQ[ 5*10^n + 3*(10^n-1)/9], Print[n]], {n, 0, 5000}]
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CROSSREFS
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Cf. A002275, A093674, A350995.
Sequence in context: A030552 A146371 A146379 * A120074 A056706 A052454
Adjacent sequences: A056711 A056712 A056713 * A056715 A056716 A056717
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KEYWORD
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nonn
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AUTHOR
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Robert G. Wilson v, Aug 11 2000
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EXTENSIONS
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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
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STATUS
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approved
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