A099047 Numbers n such that n-1 and n+1 are both composite.

5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 26, 27, 29, 31, 33, 34, 35, 37, 39, 41, 43, 45, 47, 49, 50, 51, 53, 55, 56, 57, 59, 61, 63, 64, 65, 67, 69, 71, 73, 75, 76, 77, 79, 81, 83, 85, 86, 87, 89, 91, 92, 93, 94, 95, 97, 99, 101, 103, 105, 107, 109, 111, 113, 115, 116, 117

Offset: 1

Author

Rick L. Shepherd, Nov 13 2004

Keywords

• nonn

Links

• Reinhard Zumkeller, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A010051, A002808, A104278.

Programs

Haskell

a099047 n = a099047_list !! (n-1)
a099047_list = [m | m <- [1..],
                    a010051' (m - 1) == 0 && a010051' (m + 1) == 0]
-- Reinhard Zumkeller, Aug 04 2015

Mathematica

Select[Range[6! ],!PrimeQ[ #-1]&&!PrimeQ[ #+1]&] (* Vladimir Joseph Stephan Orlovsky, Apr 26 2010 *)
Select[Range[120],AllTrue[#+{1,-1},CompositeQ]&] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, May 10 2018 *)
Mean/@SequencePosition[Table[If[CompositeQ[n],1,0],{n,120}],{1,,1}] (* Requires Mathematica version 10 or later *) (* _Harvey P. Dale, Aug 23 2019 *)

PARI

for(n=5,200,if(!isprime(n-1)&&!isprime(n+1),print1(n,",")))