%I #11 Jul 15 2021 21:25:40

%S 2,3,4,5,7,8,9,11,13,16,17,19,23,25,29,31,32,36,37,41,43,47,50,53,59,

%T 61,67,71,73,79,81,83,89,97,101,103,107,109,113,121,127,128,131,137,

%U 139,149,151,157,163,167,173,179,181,191,193,197,199,211,223,225,227

%N Numbers k such that sigma(k) - phi(k) is a prime number.

%H Charles R Greathouse IV, <a href="/A115919/b115919.txt">Table of n, a(n) for n = 1..10000</a>

%e sigma(81) - phi(81) = 67, a prime.

%t Select[Range[300],PrimeQ[DivisorSigma[1,#]-EulerPhi[#]]&] (* _Harvey P. Dale_, Feb 25 2011 *)

%o (PARI) is(n)=isprime(sigma(n)-eulerphi(n)) \\ _Charles R Greathouse IV_, Nov 27 2013

%Y Cf. A038344.

%K nonn

%O 1,1

%A _Giovanni Resta_, Feb 06 2006