login
A056682
Numbers k such that 40*R_k + 7 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
3
0, 1, 3, 9, 19, 25, 721, 1309, 3169, 28933, 66283, 67795
OFFSET
1,3
COMMENTS
Also numbers k such that (4*10^(k+1)+23)/9 is prime.
a(13) > 10^5. - Robert Price, Nov 30 2014
FORMULA
a(n) = A099413(n+1)-1. - Robert Price, Nov 30 2014
MATHEMATICA
Do[ If[ PrimeQ[40*(10^n - 1)/9 + 7], Print[n]], {n, 0, 5000}]
CROSSREFS
Sequence in context: A002091 A056259 A376220 * A032670 A068677 A069832
KEYWORD
hard,nonn
AUTHOR
Robert G. Wilson v, Aug 10 2000
EXTENSIONS
a(10)-a(12) derived from A099413 by Robert Price, Nov 30 2014
STATUS
approved