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A056682 Numbers k such that 40*R_k + 7 is prime, where R_k = 11...1 is the repunit (A002275) of length k. 3
0, 1, 3, 9, 19, 25, 721, 1309, 3169, 28933, 66283, 67795 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,3
COMMENTS
Also numbers k such that (4*10^(k+1)+23)/9 is prime.
a(13) > 10^5. - Robert Price, Nov 30 2014
LINKS
FORMULA
a(n) = A099413(n+1)-1. - Robert Price, Nov 30 2014
MATHEMATICA
Do[ If[ PrimeQ[40*(10^n - 1)/9 + 7], Print[n]], {n, 0, 5000}]
CROSSREFS
Sequence in context: A084670 A002091 A056259 * A032670 A068677 A069832
KEYWORD
hard,nonn
AUTHOR
Robert G. Wilson v, Aug 10 2000
EXTENSIONS
a(10)-a(12) derived from A099413 by Robert Price, Nov 30 2014
STATUS
approved

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Last modified March 19 03:20 EDT 2024. Contains 370952 sequences. (Running on oeis4.)