login
A103021
Numbers n such that 5*10^n + 7*R_n + 2 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
1
0, 1, 3, 7, 15, 27, 129, 189, 333, 1324, 1600, 2199, 10000, 27751
OFFSET
1,3
COMMENTS
Also numbers n such that (52*10^n+11)/9 is prime.
a(15) > 10^5. - Robert Price, Sep 05 2015
FORMULA
a(n) = A101586(n-1) + 1, for n>1.
EXAMPLE
For n=0, (52*10^n+11)/9=7 which is prime, so 0 is in the sequence.
MATHEMATICA
Do[ If[ PrimeQ[(52*10^n + 11)/9], Print[n]], {n, 0, 10000}]
CROSSREFS
Sequence in context: A343440 A147394 A350370 * A025587 A101498 A027965
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
Inserted a(1)=0 by Robert Price, Sep 05 2015
STATUS
approved