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%I #7 Mar 15 2019 12:30:10
%S 0,0,0,0,2,1,2,3,3,3,5,5,6,6,7,6,9,10,8,10,11,10,12,13,13,13,15,14,16,
%T 18,15,18,20,16,20,21,20,21,24,21,23,26,24,24,29,25,27,30,26,30,32,29,
%U 31,33,31,33,36,33,34,41,34,36,42,35,40,42,40,40,43,42,44,48,44,44
%N Number of 0 < k < n such that n-k and n+k are both nonprimes.
%F a(n) = n - 1 - A061357(n) - A291564(n).
%o (PARI) a(n)=my(s); for(c=1, n-1, s+=(bigomega(n-c)-1)*(bigomega(n+c)-1)!=0); s
%Y Cf. A061357, A291564.
%K nonn
%O 1,5
%A _Gionata Neri_, Mar 14 2019