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A099420
Numbers k such that 7*R_k - 4 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
3
1, 2, 3, 5, 9, 12, 15, 21, 264, 383, 2720, 4494, 21167, 45825, 55851, 64568, 70727
OFFSET
1,2
COMMENTS
Also numbers k such that (7*10^k - 43)/9 is prime.
a(18) > 10^5. - Robert Price, Oct 19 2014
FORMULA
a(n) = A056689(n) + 1.
MATHEMATICA
Do[ If[ PrimeQ[ 7(10^n - 1)/9 - 4], Print[n]], {n, 0, 5000}]
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Robert G. Wilson v, Oct 14 2004
EXTENSIONS
a(13)-a(17) from Robert Price, Oct 19 2014
STATUS
approved