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A056684
Numbers k such that 50*R_k + 1 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
3
OFFSET
1,1
COMMENTS
Also numbers k such that (5*10^(k+1)-41)/9 is prime.
a(4) > 10^5. - Robert Price, Nov 16 2014
FORMULA
a(n) = A099415(n) - 1. - Robert Price, Nov 16 2014
MATHEMATICA
Do[ If[ PrimeQ[50*(10^n - 1)/9 + 1], Print[n]], {n, 15000}]
CROSSREFS
Sequence in context: A262515 A041607 A042097 * A042731 A087304 A340204
KEYWORD
hard,nonn,bref,more
AUTHOR
Robert G. Wilson v, Aug 10 2000
STATUS
approved