A115919 Numbers k such that sigma(k) - phi(k) is a prime number.

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, 61, 67, 71, 73, 79, 81, 83, 89, 97, 101, 103, 107, 109, 113, 121, 127, 128, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 225, 227

Offset: 1

Author

Giovanni Resta, Feb 06 2006

Keywords

• nonn

Examples

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

Links

• Charles R Greathouse IV, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A038344.

Programs

Mathematica

Select[Range[300],PrimeQ[DivisorSigma[1,#]-EulerPhi[#]]&]  (* Harvey P. Dale, Feb 25 2011 *)

PARI

is(n)=isprime(sigma(n)-eulerphi(n)) \\ Charles R Greathouse IV, Nov 27 2013