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A056724 Numbers k such that 8*10^k + 7*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k. 1
2, 9, 15, 32, 38, 65, 123, 173, 257, 320, 326, 639, 719, 774, 902, 15210, 69999 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
Also numbers k such that (79*10^k - 7)/9 is prime.
a(18) > 10^5. - Robert Price, Jan 11 2015
LINKS
MATHEMATICA
Do[ If[ PrimeQ[ 8*10^n + 7*(10^n-1)/9], Print[n]], {n, 0, 10000}]
CROSSREFS
Sequence in context: A063094 A369154 A108463 * A295732 A083174 A067547
KEYWORD
hard,nonn
AUTHOR
Robert G. Wilson v, Aug 11 2000
EXTENSIONS
a(16)-a(17) from Robert Price, Jan 11 2015
STATUS
approved

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Last modified May 18 08:45 EDT 2024. Contains 372618 sequences. (Running on oeis4.)