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A099419
Numbers k such that 7*R_k - 6 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
3
2, 13, 20, 23, 31, 100, 241, 275, 925, 1067, 1369, 2065, 7163, 37963, 91856, 111706, 260198, 271757, 314564, 348724
OFFSET
1,1
COMMENTS
Also numbers k such that (7*10^k - 61)/9 is prime.
FORMULA
a(n) = A056688(n) + 1. - Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
MATHEMATICA
Do[ If[ PrimeQ[ 7(10^n - 1)/9 - 6], Print[n]], {n, 0, 7000}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Robert G. Wilson v, Oct 14 2004
EXTENSIONS
a(13) from Kamada link by Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
a(14)-a(15) from Erik Branger, Dec 19 2009
a(16) from Erik Branger, Jan 29 2011
a(17) from Erik Branger, Nov 26 2011
a(18) from Erik Branger, Mar 05 2012
a(19)-a(20) from Erik Branger, Sep 23 2016
STATUS
approved