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A056687
Numbers k such that 50*R_k + 9 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
2
1, 7, 11, 17, 25, 31, 137, 187, 221, 337, 1001, 2743, 6529, 38689, 39463
OFFSET
1,2
COMMENTS
Also numbers k such that (5*10^(k+1)+31)/9 is prime.
FORMULA
a(n) = A099418(n) - 1.
MATHEMATICA
Do[ If[ PrimeQ[50*(10^n - 1)/9 + 9], Print[n]], {n, 0, 5000}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Robert G. Wilson v, Aug 10 2000
EXTENSIONS
6529 from Kamada link by Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
38689 and 39463 from Serge Batalov, Jan 06 2009 confirmed as next terms by Ray Chandler, Feb 20 2012
STATUS
approved