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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

Table of n, a(n) for n=1..78.

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

Sequence in context: A246966 A081749 A074825 * A309442 A213205 A094778

Adjacent sequences:  A225060 A225061 A225062 * A225064 A225065 A225066

KEYWORD

nonn,less

AUTHOR

Juri-Stepan Gerasimov, Apr 26 2013

EXTENSIONS

Corrected by R. J. Mathar, May 04 2013

STATUS

approved

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Last modified November 17 03:06 EST 2019. Contains 329216 sequences. (Running on oeis4.)