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A259136
Numbers k such that 4*R_k + 7*10^k + 3 is prime, where R_k = 11...11 is the repunit (A002275) of length k.
0
10, 30, 120, 484, 1486, 1578, 13672, 13810, 15094, 72772, 94212
OFFSET
1,1
COMMENTS
Also, numbers k such that (67*10^k + 23)/9 is prime.
Terms from Kamada data.
a(12) > 10^5.
EXAMPLE
For k=10, 4*R_2 + 7*10^k + 3 = 4444444444 + 7000000000 + 3 = 74444444447 which is prime.
MATHEMATICA
Select[Range[0, 100000], PrimeQ[(67*10^#+23)/9] &]
CROSSREFS
Cf. A002275.
Sequence in context: A185829 A301664 A335803 * A098684 A268117 A256879
KEYWORD
nonn,more,hard
AUTHOR
Robert Price, Jun 18 2015
STATUS
approved