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A103020
Numbers n such that 5*10^n + 7*R_n - 4 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
1
1, 4, 12, 15, 18, 37, 109, 1728, 2482, 3480, 6577, 9015, 16162, 45349
OFFSET
1,2
COMMENTS
Also numbers n such that (52*10^n-43)/9 is prime.
a(15) > 10^5. - Robert Price, Sep 09 2015
FORMULA
a(n) = A101585(n) + 1.
MATHEMATICA
Do[ If[ PrimeQ[(52*10^n - 43)/9], Print[n]], {n, 0, 10000}]
CROSSREFS
Sequence in context: A260257 A256706 A340869 * A215011 A024353 A024354
KEYWORD
more,nonn
AUTHOR
Robert G. Wilson v, Jan 18 2005
EXTENSIONS
Addition of a(13) from Kamada data by Robert Price, Dec 13 2010
a(14) from Erik Branger May 01 2013 by Ray Chandler, Aug 16 2013
STATUS
approved