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A056714
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Numbers n such that 5*10^n + 3*R_n is prime, where R_n = 11...1 is the repunit (A002275) of length n.
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1
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0, 1, 3, 13, 25, 49, 143, 419, 1705, 13993, 35753, 40889
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,3
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COMMENTS
| Also numbers n such that (16*10^n-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 (f.firoozbakht(AT)sci.ui.ac.ir), Dec 02 2003
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LINKS
| Makoto Kamada, Factorizations of 533...33
Prime Puzzles, Puzzle 244
Index entries for primes involving repunits
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MATHEMATICA
| Do[ If[ PrimeQ[ 5*10^n + 3*(10^n-1)/9], Print[n]], {n, 0, 5000}]
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CROSSREFS
| Sequence in context: A030552 A146371 A146379 * A120074 A056706 A052454
Adjacent sequences: A056711 A056712 A056713 * A056715 A056716 A056717
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KEYWORD
| nonn,changed
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AUTHOR
| Robert G. Wilson v (rgwv(AT)rgwv.com), Aug 11 2000
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EXTENSIONS
| 1705 from Farideh Firoozbakht (f.firoozbakht(AT)sci.ui.ac.ir), Dec 18 2003
13993, 35753 and 40889 from Serge Batalov, Jan 2009 confirmed as next terms by Ray Chandler (rayjchandler(AT)sbcglobal.net), Feb 11 2012
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