A102707 a(n) = least k such that k*(10^n) -1 or +1, or both, is/are prime.

1, 1, 1, 2, 5, 2, 3, 2, 6, 6, 3, 11, 18, 4, 5, 6, 13, 11, 15, 3, 6, 14, 6, 6, 61, 8, 2, 2, 3, 9, 48, 48, 21, 15, 6, 30, 3, 9, 6, 18, 6, 33, 30, 3, 60, 6, 24, 33, 33, 8, 71, 31, 11, 2, 5, 3, 24, 9, 16, 39, 19, 6, 9, 53, 60, 6, 26, 3, 16, 23, 17, 19, 44, 6, 131, 56, 8, 26, 9, 42, 12, 3, 24, 41

Offset

0,4

Links

Pierre CAMI, Table of n, a(n) for n = 0..999

Example

for n=0 1*(10^0)+1=2 prime so a(0)=1
for n=1 1*(10^1)+1=11 prime so a(1)=1
for n=2 1*(10^2)+1=101 prime so a(2)=1
for n=3 2*(10^3)-1=1999 prime so a(3)=2

Mathematica

\Table[Module[{k=1}, While[NoneTrue[k 10^n+{1, -1}, PrimeQ], k++]; k], {n, 0, 90}] (* Harvey P. Dale, Jun 01 2022 *)

Crossrefs

Sequence in context: A100955 A049413 A300691 * A134486 A088006 A078311

Adjacent sequences: A102704 A102705 A102706 * A102708 A102709 A102710

Keyword

nonn

Author

Pierre CAMI, Feb 05 2005

Extensions

Initial term of b-file corrected by N. J. A. Sloane, Aug 31 2009

Definition clarified by Harvey P. Dale, Jun 01 2022

Status

approved