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A056696
Numbers k such that 90*R_k + 1 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
3
2, 4, 6, 32, 44, 104, 196, 198, 280, 300, 316, 1106, 1656, 3394, 35924, 37596, 64304, 80138, 221630
OFFSET
1,1
COMMENTS
Also numbers k such that 10^(k+1) - 9 is a prime.
FORMULA
a(n) = A095714(n) - 1.
MATHEMATICA
Do[ If[ PrimeQ[90*(10^n - 1)/9 + 1], Print[n]], {n, 0, 7000}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Robert G. Wilson v, Aug 10 2000
EXTENSIONS
a(15) - a(16) from Jason Earls, Jan 07 2008
a(17) - a(19) from Alexander Gramolin, May 13 2011
Edited by Ray Chandler, Feb 26 2012
STATUS
approved