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A099047 Numbers n such that n-1 and n+1 are both composite. 5
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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

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

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 *)

PROG

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

(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

CROSSREFS

Cf. A010051, A002808, A104278.

Sequence in context: A168146 A106505 A097985 * A174392 A084926 A049013

Adjacent sequences:  A099044 A099045 A099046 * A099048 A099049 A099050

KEYWORD

nonn

AUTHOR

Rick L. Shepherd, Nov 13 2004

STATUS

approved

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Last modified December 7 20:18 EST 2019. Contains 329847 sequences. (Running on oeis4.)