A067252 Composite n such that sigma(n)-phi(n) is prime.

4, 8, 9, 16, 25, 32, 36, 50, 81, 121, 128, 225, 256, 324, 529, 576, 625, 729, 841, 1058, 1089, 1296, 1681, 1682, 2025, 2312, 2401, 2809, 2916, 3362, 3872, 4096, 4232, 4761, 6050, 6728, 6889, 7569, 7921, 8100, 9216, 10082, 12769, 17161, 19881, 20000

Offset

1,1

Links

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

Formula

a(n) > n^2 / 2. - Charles R Greathouse IV, Nov 21 2013

Mathematica

sepQ[n_]:=!PrimeQ[n]&&PrimeQ[DivisorSigma[1, n]-EulerPhi[n]]; Select[ Range[20000], sepQ] (* Harvey P. Dale, May 02 2012 *)

Prog

(PARI) isok(n) = ! isprime(n) && isprime(sigma(n) - eulerphi(n)); \\ Michel Marcus, Nov 21 2013
(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

Crossrefs

Subsequence of A028982.

Sequence in context: A155568 A353485 A371010 * A381075 A348995 A324723

Adjacent sequences: A067249 A067250 A067251 * A067253 A067254 A067255

Keyword

nonn

Author

Benoit Cloitre, Feb 20 2002

Extensions

Corrected by Harvey P. Dale, May 02 2012

Status

approved