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A056695
Numbers k such that 80*R_k + 7 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
2
0, 2, 3, 5, 8, 11, 71, 117, 123, 189, 243, 303, 356, 1421, 2690, 5537, 7580, 21905, 32175, 44357
OFFSET
1,2
COMMENTS
Also numbers k such that (8*10^(k+1)-17)/9 is prime.
a(21) > 10^5. - Robert Price, May 20 2014
FORMULA
a(n) = A096846(n) - 1.
MATHEMATICA
Do[ If[ PrimeQ[80*(10^n - 1)/9 + 7], Print[n]], {n, 0, 5000}]
CROSSREFS
KEYWORD
hard,nonn,more
AUTHOR
Robert G. Wilson v, Aug 10 2000
EXTENSIONS
More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
a(18)-a(20) derived from A096846 by Robert Price, May 20 2014
STATUS
approved