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A259134
Numbers k such that 9*R_k + 7*10^k - 2 is prime, where R_k = 11...11 is the repunit (A002275) of length k.
0
0, 2, 4, 28, 156, 322, 352, 1212, 1284, 7984, 15192, 84772, 119930, 148860
OFFSET
1,2
COMMENTS
Also, numbers k such that 8*10^k - 3 is prime.
Terms from Kamada data.
Note that Kamada does not recognize k=0 as 5 is a degenerate case of form ABB..BBA.
a(15) > 2*10^5.
EXAMPLE
For k=2, 9*R_2 + 7*10^k - 2 = 99 + 700 - 2 = 797 which is prime.
MATHEMATICA
Select[Range[0, 200000], PrimeQ[8*10^#-3] &]
CROSSREFS
Cf. A002275.
Sequence in context: A264667 A356700 A357158 * A117443 A095858 A062792
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jun 18 2015
STATUS
approved