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A084832
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Numbers n such that 2*R_n - 1 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
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1
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OFFSET
| 1,1
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COMMENTS
| Also numbers n such that (2*10^n-11)/9 is prime.
Larger values not certified prime.
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LINKS
| Index entries for primes involving repunits
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EXAMPLE
| a(1)=4 because 2*(10^4-1)/9-1 = 2221 is prime.
n=18 means that 222222222222222221 is prime.
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MATHEMATICA
| Do[ If[ PrimeQ[(10^n - 1)/9 + 6], Print[n]], {n, 0, 7000}] (from Robert G. Wilson v Oct 14 2004)
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CROSSREFS
| Cf. A084831, A096503, A096504, A096505, A096506, A096507, A096508, A096841, A096842, A096843, A096844, A096845, A096846, A000203.
Sequence in context: A020072 A020027 A197593 * A135177 A137958 A201826
Adjacent sequences: A084829 A084830 A084831 * A084833 A084834 A084835
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KEYWORD
| more,nonn
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AUTHOR
| Jason Earls (zevi_35711(AT)yahoo.com), Jun 05 2003
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EXTENSIONS
| a(8) from Labos E. (labos(AT)ana.sote.hu), Jul 15 2004
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