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A103106
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Numbers n such that 9*10^n + 7*R_n - 4 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
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1
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0, 6, 15, 36, 45, 447, 1031, 1239, 3492, 6069, 13647, 17238, 56271, 72711, 75381
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OFFSET
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1,2
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COMMENTS
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Also numbers n such that (88*10^n-43)/9 is prime.
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LINKS
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FORMULA
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MATHEMATICA
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Do[ If[ PrimeQ[(88*10^n - 43)/9], Print[n]], {n, 0, 10000}]
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PROG
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(PARI) is(n)=isprime(9*10^n + 7*((10^n-1)/9) - 4) \\ Anders Hellström, Nov 12 2015
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CROSSREFS
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KEYWORD
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more,nonn
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AUTHOR
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EXTENSIONS
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Inserted a(1)=0 and added a(13)-a(15) from Robert Price, Nov 12 2015
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STATUS
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approved
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