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A056680 Numbers k such that 30*R_k + 7 is prime, where R_k = 11...1 is the repunit (A002275) of length k. 2
0, 1, 2, 5, 45, 393, 977, 2585, 2810, 2967, 3641, 4826, 4917, 5591, 5705, 10682, 12890, 14117, 74349, 88679 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,3
COMMENTS
Also numbers k such that (10^(k+1)+11)/3 is prime.
a(21) > 10^5. - Robert Price, Nov 02 2014
LINKS
FORMULA
a(n) = A099411(n) - 1. - Robert Price, Nov 02 2014
MATHEMATICA
Do[ If[ PrimeQ[30*(10^n - 1)/9 + 7], Print[n]], {n, 0, 10000}]
CROSSREFS
Sequence in context: A088309 A334252 A307147 * A005166 A121621 A225147
KEYWORD
hard,nonn
AUTHOR
Robert G. Wilson v, Aug 10 2000
EXTENSIONS
a(16)-a(20) derived from A099411 by Robert Price, Nov 02 2014
STATUS
approved

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Last modified March 18 22:56 EDT 2024. Contains 370952 sequences. (Running on oeis4.)