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A103054
Numbers n such that 7*10^n + 2*R_n + 7 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
1
1, 3, 4, 7, 9, 13, 33, 37, 76, 147, 297, 1851, 3457, 3609, 45219
OFFSET
1,2
COMMENTS
Also numbers n such that (65*10^n+61)/9 is prime.
a(16) > 10^5. - Robert Price, Oct 01 2015
FORMULA
a(n) = A101136(n) + 1.
MATHEMATICA
Do[ If[ PrimeQ[(65*10^n + 61)/9], Print[n]], {n, 0, 10000}]
Select[Range[10000], PrimeQ[(65 10^# + 61)/9] &] (* Vincenzo Librandi, Oct 02 2015 *)
PROG
(Magma) [n: n in [0..3*10^2]| IsPrime((65*10^n+61) div 9)]; // Vincenzo Librandi, Oct 02 2015
(PARI) for(n=1, 1e4, if (isprime((65*10^n+61)/9), print1(n", "))) \\ Altug Alkan, Oct 02 2015
CROSSREFS
Sequence in context: A330146 A295069 A349360 * A140208 A098390 A266769
KEYWORD
more,nonn
AUTHOR
Robert G. Wilson v, Jan 19 2005
EXTENSIONS
a(15) from Erik Branger May 01 2013 by Ray Chandler, Aug 16 2013
STATUS
approved