1,2

Table of n, a(n) for n=1..75.

a(1)=1 because 2*1-1=1 is nonprime and 2*1+1=3 is prime.

Select[Range[150], Sort[PrimeQ[2 #+{1, -1}]]=={False, True}&] (* Harvey P. Dale, Aug 21 2013 *)

Sequence in context: A276705 A168044 A248188 * A285085 A231507 A097482

Adjacent sequences: A174044 A174045 A174046 * A174048 A174049 A174050

nonn,easy,less

Juri-Stepan Gerasimov, Mar 06 2010

Corrected by Charles R Greathouse IV, Mar 23 2010

approved