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A098088
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Numbers n such that 6*R_n - 5 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
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0
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2, 3, 4, 10, 18, 21, 22, 28, 43, 66, 121, 133, 178, 241, 454, 553, 1600, 2175, 2978, 3649, 7708, 8316, 10392, 12458, 21057, 26223
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,1
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COMMENTS
| Also numbers n such that (2*10^n-17)/3 is prime.
The terms 1600, 2175, 2978 and 3649 correspond to primes. - Joao da Silva (zxawyh66(AT)yahoo.com), Oct 3 2005
No others less than 30000.
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LINKS
| Makoto Kamada, Factorizations of 66...661.
Index entries for primes involving repunits
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FORMULA
| a(n) = A056658(n) + 1. - Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
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EXAMPLE
| If n = 4 we get ((2*10^4)-17)/3 = 19983/3 = 6661, which is prime.
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MATHEMATICA
| Do[ If[ PrimeQ[ 2(10^n - 1)/3 - 5], Print[n]], {n, 0, 7000}]
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CROSSREFS
| Sequence in context: A085934 A056701 A055506 * A080500 A007661 A049891
Adjacent sequences: A098085 A098086 A098087 * A098089 A098090 A098091
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KEYWORD
| more,nonn
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AUTHOR
| Julien Peter Benney (jpbenney(AT)ftml.net), Sep 14 2004
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EXTENSIONS
| a(21)-a(22) from Kamada link by Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
a(23)-a(26) from Kamada link by Ray Chandler (rayjchandler(AT)sbcglobal.net), Dec 23 2010
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