|
| |
|
|
A225063
|
|
Smallest k such that (k-1)*prime(n) +/- k are both prime.
|
|
1
|
|
|
|
5, 4, 2, 3, 4, 3, 3, 4, 15, 6, 4, 6, 6, 3, 6, 3, 4, 7, 3, 7, 18, 4, 6, 4, 3, 7, 6, 25, 7, 3, 3, 4, 3, 31, 6, 4, 3, 6, 3, 13, 10, 12, 4, 3, 13, 4, 6, 3, 21, 4, 43, 10, 4, 9, 6, 10, 7, 21, 28, 19, 3, 6, 13, 4, 6, 33, 7, 15, 28, 19, 10, 6, 18, 18, 6, 21, 4, 36
(list;
graph;
refs;
listen;
history;
text;
internal format)
|
|
|
|
OFFSET
|
1,1
|
|
|
LINKS
|
|
|
|
EXAMPLE
|
a(1) = 5 because (5-1)*2 - 5 = 3 and (5-1)*2 + 5 = 13 are both prime;
a(2) = 4 because (4-1)*3 - 4 = 5 and (4-1)*3 + 4 = 13 are both prime;
a(3) = 2 because (2-1)*5 - 2 = 3 and (2-1)*5 + 2 = 7 are both prime;
a(4) = 3 because (3-1)*7 - 3 = 11 and (3-1)*7 + 3 = 17 are both prime;
a(5) = 4 because (4-1)*11 - 4 = 29 and (4-1)*11 + 4 = 37 are both prime;
a(6) = 3 because (3-1)*13 - 3 = 23 and (3-1)*13 + 3 = 29 are botn prime;
a(7) = 3 because (3-1)*17 - 3 = 31 and (3-1)*17 + 3 = 37 are both prime;
a(8) = 4 because (4-1)*19 - 4 = 53 and (4-1)*19 + 4 = 61 are both prime;
a(9) = 15 because (15-1)*23 - 15 = 307 and (15-1)*23 + 15 = 337 are both prime.
|
|
|
MATHEMATICA
|
sk[n_]:=Module[{k=2}, While[!PrimeQ[(k-1)n+k]||!PrimeQ[(k-1)n-k], k++]; k]; sk/@Prime[Range[80]] (* Harvey P. Dale, Oct 04 2015 *)
|
|
|
CROSSREFS
|
|
|
|
KEYWORD
|
nonn,less
|
|
|
AUTHOR
|
|
|
|
EXTENSIONS
|
|
|
|
STATUS
|
approved
|
| |
|
|
