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A056688
Numbers k such that 70*R_k + 1 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
2
1, 12, 19, 22, 30, 99, 240, 274, 924, 1066, 1368, 2064, 7162, 37962, 91855, 111705, 260197, 271756, 314563, 348723
OFFSET
1,2
COMMENTS
Also numbers k such that (7*10^(k+1)-61)/9 is prime.
FORMULA
a(n) = A099419(n) - 1.
MATHEMATICA
Do[ If[ PrimeQ[70*(10^n - 1)/9 + 1], Print[n]], {n, 0, 7000}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Robert G. Wilson v, Aug 10 2000
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