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A056715 Numbers k such that 5*10^k + 7*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k. 1
0, 2, 8, 14, 17, 18, 33, 35, 126, 183, 324, 344, 866, 992, 1226, 2355, 13344, 75027, 77322 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,2
COMMENTS
Also numbers n such that (52*10^k - 7)/9 is prime.
a(20) > 10^5. - Robert Price, Dec 30 2014
LINKS
MATHEMATICA
Do[ If[ PrimeQ[ 5*10^n + 7*(10^n-1)/9], Print[n]], {n, 0, 5000}]
CROSSREFS
Sequence in context: A333968 A194278 A050619 * A160556 A105610 A324691
KEYWORD
hard,nonn
AUTHOR
Robert G. Wilson v, Aug 11 2000
EXTENSIONS
a(17)-a(19) from Robert Price, Dec 30 2014
STATUS
approved

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Last modified May 18 08:25 EDT 2024. Contains 372618 sequences. (Running on oeis4.)