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A103080
Numbers n such that 8*10^n + 4*R_n - 1 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
1
0, 1, 3, 4, 10, 15, 25, 69, 159, 166, 261, 442, 1339, 1797, 2170, 3163, 3472, 4917, 18267
OFFSET
1,3
COMMENTS
Also numbers n such that (76*10^n-13)/9 is prime.
a(20) > 10^5. - Robert Price, Oct 20 2015
FORMULA
a(n) = A101067(n-1) + 1, for n>1.
MATHEMATICA
Do[ If[ PrimeQ[(76*10^n - 13)/9], Print[n]], {n, 0, 10000}]
CROSSREFS
Sequence in context: A242342 A204292 A218277 * A055720 A054184 A307057
KEYWORD
more,nonn
AUTHOR
Robert G. Wilson v, Jan 19 2005
EXTENSIONS
a(18) from Kamada data by Robert Price, Dec 14 2010
Inserted a(1)=0 by Robert Price, Oct 20 2015
STATUS
approved