A099049 Numbers n such that n-1 and n+1 are both prime or both composite.

4, 5, 6, 7, 9, 11, 12, 13, 15, 17, 18, 19, 21, 23, 25, 26, 27, 29, 30, 31, 33, 34, 35, 37, 39, 41, 42, 43, 45, 47, 49, 50, 51, 53, 55, 56, 57, 59, 60, 61, 63, 64, 65, 67, 69, 71, 72, 73, 75, 76, 77, 79, 81, 83, 85, 86, 87, 89, 91, 92, 93, 94, 95, 97, 99, 101, 102, 103, 105, 107

Offset: 1

Author

Rick L. Shepherd, Nov 13 2004

Keywords

• nonn

Programs

Mathematica

Select[Range[120],AllTrue[#+{1,-1},CompositeQ]||AllTrue[#+{1,-1},PrimeQ]&] (* Harvey P. Dale, Sep 01 2021 *)

PARI

for(n=3,200,s=isprime(n-1)+isprime(n+1);if(s<>1,print1(n,",")))