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A056656
Numbers k such that 20*R_k + 3 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
2
0, 1, 2, 7, 10, 35, 94, 100, 127, 259, 350, 466, 644, 1010, 1177, 1216, 2441, 3760, 3805, 15616, 26458, 63116, 88544, 93496
OFFSET
1,3
COMMENTS
Also numbers k such that (2*10^(k+1)+7)/9 is prime.
FORMULA
a(n) = A096506(n) - 1.
MATHEMATICA
Do[ If[ PrimeQ[ 20*(10^n - 1)/9 + 3 ], Print[n]], {n, 7000}]
CROSSREFS
Cf. A093162 (corresponding primes), A096506.
Sequence in context: A084184 A015963 A293002 * A351240 A151346 A247166
KEYWORD
nonn
AUTHOR
Robert G. Wilson v, Aug 09 2000
EXTENSIONS
2441 from Rick L. Shepherd, Mar 27 2004
15616 and 26458 from Erik Branger, Jan 31 2010
63116, 88544 and 93496 from Erik Branger, Mar 14 2011; confirmed as next terms by Ray Chandler, Feb 17 2012
STATUS
approved