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A256828
Numbers k such that 7*R_k - 40 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
0
2, 6, 18, 48, 204, 10386, 19524, 28320
OFFSET
1,1
COMMENTS
Also, numbers k such that (7*10^k - 367)/9 is prime.
Terms from Kamada.
a(9) > 30000.
a(9) > 2*10^5. - Tyler Busby, Feb 02 2023
EXAMPLE
For k=6, 7*R_6 - 40 = 777777 - 40 = 777737 which is prime.
MATHEMATICA
Select[Range[0, 250000], PrimeQ[(7*10^# - 367)/9] &]
PROG
(Magma) [n: n in [2..300] | IsPrime((7*10^n-367) div 9)]; // Vincenzo Librandi, Apr 11 2015
CROSSREFS
Cf. A002275.
Sequence in context: A295499 A059413 A377118 * A197055 A258625 A062026
KEYWORD
more,hard,nonn
AUTHOR
Robert Price, Apr 10 2015
STATUS
approved