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A055520
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Numbers k such that 30*R_k + 1 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
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3
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1, 2, 3, 4, 5, 6, 7, 17, 39, 49, 59, 77, 100, 150, 318, 381, 783, 1731, 1917, 8854, 11244, 11959, 12129, 18532, 22717, 23364, 24252, 24548, 25323, 30177, 53717, 380975, 424860
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OFFSET
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1,2
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COMMENTS
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Also numbers k such that (10^(k+1)-7)/3 is prime.
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REFERENCES
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Martin Gardner, Strong Laws of Small Primes, in The Last Recreations, p. 194 (1997).
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LINKS
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Eric Weisstein's World of Mathematics, 3
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FORMULA
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MATHEMATICA
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Do[ If[ PrimeQ[30*(10^n - 1)/9 + 1], Print[n]], {n, 0, 50410}]
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CROSSREFS
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KEYWORD
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hard,nonn
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AUTHOR
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EXTENSIONS
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a(32)-a(33) from Leonid Durman, Jan 09-10 2012
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STATUS
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approved
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