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A056727 Numbers k such that 9*10^k + 7*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k. 1
1, 2, 4, 19, 28, 73, 203, 220, 274, 292, 470, 763, 1891, 3307, 7007, 7306, 9755, 11395, 39452, 78242 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,2
COMMENTS
Also numbers k such that (88*10^k - 7)/9 is prime.
a(21) > 10^5. - Robert Price, Nov 30 2014
LINKS
MATHEMATICA
Do[ If[ PrimeQ[ 9*10^n + 7*(10^n-1)/9], Print[n]], {n, 0, 10000}]
CROSSREFS
Cf. A002275, A093944 (corresponding primes).
Sequence in context: A268338 A171735 A201379 * A154140 A131578 A018273
KEYWORD
hard,nonn
AUTHOR
Robert G. Wilson v, Aug 11 2000
EXTENSIONS
a(18)-a(20) from Robert Price, Nov 30 2014
STATUS
approved

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Last modified June 17 12:36 EDT 2024. Contains 373445 sequences. (Running on oeis4.)