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A056713
Numbers k such that 5*10^k + R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
1
0, 5, 12, 15, 84, 144, 150, 1235, 1727, 1812, 8687, 12390, 28608, 42959, 51111, 96798, 99143
OFFSET
1,2
COMMENTS
Also numbers k such that (46*10^k - 1)/9 is prime.
a(18) > 10^5. - Robert Price, Dec 08 2014
MATHEMATICA
Do[ If[ PrimeQ[ 5*10^n + (10^n-1)/9], Print[n]], {n, 0, 10000}]
CROSSREFS
Sequence in context: A184399 A314275 A314276 * A110670 A111523 A078231
KEYWORD
hard,nonn
AUTHOR
Robert G. Wilson v, Aug 11 2000
EXTENSIONS
a(11) from Hugo Pfoertner, Oct 19 2004
a(12)-a(17) from Robert Price, Dec 08 2014
STATUS
approved