|
|
A257037
|
|
Numbers k such that 9*R_(k+2) - 7*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
|
|
0
|
|
|
1, 8, 9, 14, 54, 80, 487, 551, 600, 2502, 2544, 5593, 7949, 8635, 13407, 31128, 45504, 45933, 52303, 65121, 167501, 359354, 642225, 1029523, 1170023
(list;
graph;
refs;
listen;
history;
text;
internal format)
|
|
|
OFFSET
|
1,2
|
|
COMMENTS
|
Also, numbers k such that 93*10^k - 1 is prime.
Terms up to a(22) from Kamada.
|
|
LINKS
|
|
|
EXAMPLE
|
For k = 8, 9*R_10 - 7*10^8 = 9999999999 - 700000000 = 9299999999 which is prime, so 8 is a term.
|
|
MATHEMATICA
|
Select[Range[0, 400000], PrimeQ[93*10^#-1 ] &]
|
|
PROG
|
|
|
CROSSREFS
|
|
|
KEYWORD
|
more,hard,nonn
|
|
AUTHOR
|
|
|
EXTENSIONS
|
|
|
STATUS
|
approved
|
|
|
|