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A089675
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Numbers n such that 9*R_n - 2 is a prime number, where R_n = 11...1 is the repunit (A002275) of length n.
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15
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1, 2, 3, 17, 140, 990, 1887, 3530, 5996, 13820, 21873, 26045, 87720, 232599
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,2
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COMMENTS
| Also numbers n such that 10^n - 3 is prime.
If n is in the sequence (10^n-3 is prime) and m=3*(10^n-3) then phi(m)=reversal(m), i.e., m is in the sequence A069215. - Farideh Firoozbakht (mymontain(AT)yahoo.com), Dec 25 2004.
No further terms for n<=407197, see Kamada link.
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LINKS
| Makoto Kamada, Factorizations of 99...997. [From M. F. Hasler, Dec 04 2010]
Index entries for primes involving repunits.
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EXAMPLE
| 10^2 - 3 = 97 is a prime number (in fact all terms are the largest less than 10^n).
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MATHEMATICA
| m = 1000; For[n = 1, n < m, If[PrimeQ[10^n - 3], Print[n]]; n++]
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CROSSREFS
| Cf. A069215, A101700.
Sequence in context: A135726 A164816 A042978 * A041383 A042903 A132534
Adjacent sequences: A089672 A089673 A089674 * A089676 A089677 A089678
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KEYWORD
| more,nonn
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AUTHOR
| Michael Gottlieb (mzrg(AT)verizon.net), Jan 05 2004.
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EXTENSIONS
| a(8) from Robert G. Wilson v, Jan 14 2004.
a(9) and a(10) from Gabriel Cunningham (gcasey(AT)mit.edu), Mar 06 2004.
a(11) from Gabriel Cunningham (gcasey(AT)mit.edu), Mar 13 2004.
a(12) from Henri Lifchitz (HLifchitz(AT)compuserve.com).
Edited by Patrick De Geest (pdg(AT)worldofnumbers.com), Dec 28 2004.
Edited by Ray Chandler (rayjchandler(AT)sbcglobal.net), Dec 23 2010
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