A115919 - OEIS

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

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,1

LINKS

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

EXAMPLE

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

MATHEMATICA

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

PROG

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

CROSSREFS