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A256906 Numbers k such that 7*R_k + 10 is prime, where R_k = 11...1 is the repunit (A002275) of length k. 0
1, 3, 6, 15, 21, 24, 31, 291, 408, 457, 643, 2671, 2676, 10893, 21151, 26445, 77304, 96312 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,2
COMMENTS
Also, numbers k such that (7*10^k + 83)/9 is prime.
Terms from Kamada. Note Kamada does not recognize k=1 as 17 is a degenerate case of form AAA..ABA.
a(17) > 30000.
a(19) > 2*10^5. - Tyler Busby, Feb 01 2023
LINKS
EXAMPLE
For k=3, 7*R_11 + 10 = 777 + 10 = 787 which is prime.
MATHEMATICA
Select[Range[0, 30000], PrimeQ[(7*10^# + 83)/9] &]
CROSSREFS
Cf. A002275.
Sequence in context: A289296 A310126 A298015 * A095869 A292610 A069559
KEYWORD
more,hard,nonn
AUTHOR
Robert Price, Apr 12 2015
EXTENSIONS
a(17)-a(18) from Tyler Busby, Feb 01 2023
STATUS
approved

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Last modified July 13 23:31 EDT 2024. Contains 374290 sequences. (Running on oeis4.)