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%I #15 Aug 23 2019 14:25:00
%S 5,7,9,11,13,15,17,19,21,23,25,26,27,29,31,33,34,35,37,39,41,43,45,47,
%T 49,50,51,53,55,56,57,59,61,63,64,65,67,69,71,73,75,76,77,79,81,83,85,
%U 86,87,89,91,92,93,94,95,97,99,101,103,105,107,109,111,113,115,116,117
%N Numbers n such that n-1 and n+1 are both composite.
%H Reinhard Zumkeller, <a href="/A099047/b099047.txt">Table of n, a(n) for n = 1..10000</a>
%t Select[Range[6! ],!PrimeQ[ #-1]&&!PrimeQ[ #+1]&] (* _Vladimir Joseph Stephan Orlovsky_, Apr 26 2010 *)
%t Select[Range[120],AllTrue[#+{1,-1},CompositeQ]&] (* Requires Mathematica version 10 or later *) (* _Harvey P. Dale_, May 10 2018 *)
%t Mean/@SequencePosition[Table[If[CompositeQ[n],1,0],{n,120}],{1,_,1}] (* Requires Mathematica version 10 or later *) (* _Harvey P. Dale_, Aug 23 2019 *)
%o (PARI) for(n=5,200,if(!isprime(n-1)&&!isprime(n+1),print1(n,",")))
%o (Haskell)
%o a099047 n = a099047_list !! (n-1)
%o a099047_list = [m | m <- [1..],
%o a010051' (m - 1) == 0 && a010051' (m + 1) == 0]
%o -- _Reinhard Zumkeller_, Aug 04 2015
%Y Cf. A010051, A002808, A104278.
%K nonn
%O 1,1
%A _Rick L. Shepherd_, Nov 13 2004
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