A171845 Numbers n >= 0 such that n-1 and n+1 are both primes or both nonprimes, but excluding primes.

0, 4, 6, 9, 12, 15, 18, 21, 25, 26, 27, 30, 33, 34, 35, 39, 42, 45, 49, 50, 51, 55, 56, 57, 60, 63, 64, 65, 69, 72, 75, 76, 77, 81, 85, 86, 87, 91, 92, 93, 94, 95, 99, 102, 105, 108, 111, 115, 116, 117, 118, 119, 120, 121, 122, 123, 124, 125, 129, 133, 134, 135, 138

Offset

1,2

Links

Harvey P. Dale, Table of n, a(n) for n = 1..2000

Maple

a:=[];
for k from 0 to 5000 do
if not isprime(k) then
   if (isprime(k-1) and isprime(k+1)) or
   (not isprime(k-1) and not isprime(k+1)) then a:=[op(a), k]; fi; fi; od:
a; # N. J. A. Sloane, Sep 23 2019

Mathematica

bpQ[n_]:=!PrimeQ[n]&&EvenQ[Total[If[PrimeQ[#], 1, 0]&/@{n-1, n+1}]]; Select[ Range[ 0, 150], bpQ] (* Harvey P. Dale, Sep 23 2019 *)

Crossrefs

Sequence in context: A007074 A054087 A079255 * A157124 A162735 A024675

Adjacent sequences: A171842 A171843 A171844 * A171846 A171847 A171848

Keyword

nonn,easy

Author

Juri-Stepan Gerasimov, Jan 21 2010

Extensions

Edited by Charles R Greathouse IV, Mar 23 2010

Definition and data corrected by N. J. A. Sloane, Sep 23 2019. Thanks to Harvey P. Dale for pointing that there were errors. - N. J. A. Sloane, Sep 23 2019

Status

approved