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URL: https://oeis.org/A056657

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A056657
Numbers k such that 60*R_k + 7 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
6
0, 1, 5, 7, 8, 10, 19, 22, 40, 62, 65, 118, 121, 148, 251, 283, 304, 591, 745, 874, 1203, 1363, 2239, 2402, 5105, 5775, 5812, 12455, 14234, 39605, 55543, 84238, 275921
OFFSET
1,3
COMMENTS
Also numbers k such that (20*10^k+1)/3 is prime.
FORMULA
a(n) = A096507(n) - 1.
EXAMPLE
7, 67, 666667, 66666667, 666666667, 66666666667, etc. are primes.
MATHEMATICA
Do[ If[ PrimeQ[ 60*(10^n - 1)/9 + 7 ], Print[n]], {n, 25556}]
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
Robert G. Wilson v, Aug 09 2000
EXTENSIONS
More terms from Robert G. Wilson v, Oct 22 2003
2239,2402,5105,5775 from Farideh Firoozbakht, Dec 23 2003
39605 and 55543 from Serge Batalov, Jun 2009
84238 from Serge Batalov, Jul 06 2009 confirmed as next term by Ray Chandler, Feb 23 2012
a(33) derived from A096507 by Robert Price, Jul 07 2024
STATUS
approved