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A256725
Numbers k such that R_k + 40 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
0
1, 3, 4, 7, 60, 394, 552, 1164, 1494, 5398, 7899, 11254, 13224, 77637, 118324, 120574, 142425, 142699, 157792, 188164
OFFSET
1,2
COMMENTS
Also, numbers k such that (10^k + 359)/9 is prime.
Terms from Kamada data. Note that Kamada does not recognize k=1 as 41 is a degenerate case of form AAA..ABA.
a(21) > 10^6.
EXAMPLE
For k=3, R_3 + 40 = 111 + 40 = 151 which is prime.
MATHEMATICA
Select[Range[0, 250000], PrimeQ[(10^# + 359)/9] &]
PROG
(Magma) [n: n in [1..400] | IsPrime((10^n+359) div 9)]; // Vincenzo Librandi, Apr 10 2015
CROSSREFS
Cf. A002275.
Sequence in context: A041379 A242859 A134471 * A041463 A041863 A042543
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Apr 09 2015
STATUS
approved