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%I #14 Nov 21 2013 12:47:35
%S 4,8,9,16,25,32,36,50,81,121,128,225,256,324,529,576,625,729,841,1058,
%T 1089,1296,1681,1682,2025,2312,2401,2809,2916,3362,3872,4096,4232,
%U 4761,6050,6728,6889,7569,7921,8100,9216,10082,12769,17161,19881,20000
%N Composite n such that sigma(n)-phi(n) is prime.
%H Charles R Greathouse IV, <a href="/A067252/b067252.txt">Table of n, a(n) for n = 1..10000</a>
%F a(n) > n^2 / 2. - _Charles R Greathouse IV_, Nov 21 2013
%t sepQ[n_]:=!PrimeQ[n]&&PrimeQ[DivisorSigma[1,n]-EulerPhi[n]]; Select[ Range[20000],sepQ] (* _Harvey P. Dale_, May 02 2012 *)
%o (PARI) isok(n) = ! isprime(n) && isprime(sigma(n) - eulerphi(n)); \\ _Michel Marcus_, Nov 21 2013
%o (PARI) list(lim)=my(v=List(),f);for(n=2,sqrtint(lim\2),f=factor(2*n^2);if(isprime(sigma(f)-eulerphi(f)),listput(v,2*n^2)));for(n=2,sqrtint(lim\1),f=factor(n^2);if(isprime(sigma(f)-eulerphi(f)),listput(v,n^2)));Set(v) \\ _Charles R Greathouse IV_, Nov 21 2013
%Y Subsequence of A028982.
%K nonn
%O 1,1
%A _Benoit Cloitre_, Feb 20 2002
%E Corrected by Harvey P. Dale, May 02 2012
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