A173136 Odd nonprimes n such that exactly one of 2n-1 and 2n+1 is prime.
1, 27, 33, 35, 39, 45, 49, 55, 57, 63, 65, 81, 87, 91, 95, 105, 111, 115, 117, 119, 121, 125, 129, 147, 153, 155, 159, 165, 169, 175, 177, 183, 187, 189, 195, 201, 205, 209, 215, 217, 219, 221, 225, 243, 245, 249, 255, 273, 279, 289, 297, 299, 301, 303, 315
Offset: 1
Examples
a(1)=1 because 2*1-1=1 is nonprime and 2*1+1=3 is prime.
Links
- Harvey P. Dale, Table of n, a(n) for n = 1..1000
Programs
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Mathematica
Select[Range[1,341,2],!PrimeQ[#]&&Total[Boole[PrimeQ[2 #+{1,-1}]]]==1&] (* Harvey P. Dale, Sep 01 2017 *)
Extensions
Corrected at 3 or more places by R. J. Mathar, Mar 09 2010