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A056686
Numbers k such that 50*R_k + 7 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
2
0, 2, 3, 5, 9, 14, 21, 87, 206, 527, 959, 2099, 18521, 36782, 38516
OFFSET
1,2
COMMENTS
Also numbers k such that (5*10^(k+1)+13)/9 is prime.
a(16) > 10^5. - Robert Price, Nov 01 2014
FORMULA
a(n) = A099417(n+1) - 1. - Robert Price, Nov 01 2014
MATHEMATICA
Do[ If[ PrimeQ[50*(10^n - 1)/9 + 7], Print[n]], {n, 0, 5000}]
CROSSREFS
Sequence in context: A076027 A280204 A240841 * A326332 A079962 A244986
KEYWORD
hard,nonn
AUTHOR
Robert G. Wilson v, Aug 10 2000
EXTENSIONS
a(13)-a(15) derived from A099417 by Robert Price, Nov 01 2014
STATUS
approved