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A098088
Numbers k such that 6*R_k - 5 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
2
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, 48297, 64041, 84904, 92976, 95072, 103161, 140461, 141751, 150612, 265321
OFFSET
1,1
COMMENTS
Also numbers k such that (2*10^k - 17)/3 is prime.
The terms 1600, 2175, 2978 and 3649 correspond to primes. - Joao da Silva (zxawyh66(AT)yahoo.com), Oct 03 2005
a(37) > 3*10^5, Robert Price, Oct 19 2023
FORMULA
a(n) = A056658(n) + 1. - Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
EXAMPLE
If n = 4 we get ((2*10^4)-17)/3 = 19983/3 = 6661, which is prime.
MATHEMATICA
Do[ If[ PrimeQ[ 2(10^n - 1)/3 - 5], Print[n]], {n, 0, 7000}]
CROSSREFS
Sequence in context: A285190 A055506 A329660 * A080500 A007661 A049891
KEYWORD
more,nonn
AUTHOR
Julien Peter Benney (jpbenney(AT)ftml.net), Sep 14 2004
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, Dec 23 2010
a(27) from Kamada link by Robert Price, Aug 17 2014
a(28)-a(31) from Robert Price, Aug 17 2014
a(32)-a(36) from Robert Price, Oct 19 2023
STATUS
approved