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A099190
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Numbers n such that 8*10^n-7 is prime.
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5
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1, 3, 5, 6, 10, 12, 13, 39, 48, 54, 64, 82, 147, 148, 360, 399, 1638, 1876, 2146, 2194, 15789, 23074, 38466, 68400
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,2
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COMMENTS
| Also numbers n such that 7*10^n + 9*R_n - 6 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
Primes of the form 7*10^n+9R_n-6 are the only primes which are one more than twice their reversal.
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LINKS
| Makoto Kamada, Factorizations of 799...993.
Erich Friedman, What's Special About This Number?.
Index entries for primes involving repunits.
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EXAMPLE
| 73, 7993, 799993, 7999993, etc. are primes.
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MATHEMATICA
| Do[ If[ PrimeQ[8*10^n - 7], Print[n]], {n, 10000}]
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CROSSREFS
| Cf. A099181.
Sequence in context: A072282 A047327 A028927 * A122772 A140449 A207063
Adjacent sequences: A099187 A099188 A099189 * A099191 A099192 A099193
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KEYWORD
| more,nonn
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AUTHOR
| Robert G. Wilson v (rgwv(AT)rgwv.com), Oct 01 2004
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EXTENSIONS
| a(19) & a(20) from Robert G. Wilson v (rgwv(AT)rgwv.com), Jan 19 2005
a(21)-a(24) from Kamada data by Robert Price (pamandbobprice(AT)yahoo.com), Dec 14 2010
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