OFFSET
1,1
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
Juri-Stepan Gerasimov, Apr 26 2013
EXTENSIONS
Corrected by R. J. Mathar, May 04 2013
STATUS
approved