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A096508
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Numbers n for which 8*R_n + 1 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
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20
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2, 14, 17, 35, 4175, 4472, 9812, 12260, 12341, 13760, 14576, 53411, 144683, 148328
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OFFSET
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1,1
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COMMENTS
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Also numbers n such that (8*10^n+1)/9 is prime.
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LINKS
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FORMULA
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EXAMPLE
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a(4) = 35 means that 88888888888888888888888888888888889 (34 8's) is a prime number.
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MAPLE
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select(n -> isprime((8*10^n+1)/9), [$1..10000]); # Robert Israel, Sep 07 2014
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MATHEMATICA
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Do[ If[ PrimeQ[ 8(10^n - 1)/9 + 1], Print[n]], {n, 0, 30000}] (* Robert G. Wilson v, Oct 15 2004 *)
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PROG
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(PARI)
for(n=1, 10^4, if(ispseudoprime(8*(10^n-1)/9+1), print1(n, ", "))) \\ Derek Orr, Sep 06 2014
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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Added four missing terms (9812, 12260, 12341, 13760), added a(12)-a(14) from Kamada data by Robert Price, Sep 06 2014
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STATUS
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approved
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