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A067252 Composite n such that sigma(n)-phi(n) is prime. 1
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 (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

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: A062559 A010417 A155568 * A324723 A272758 A227645

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

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Last modified December 7 00:16 EST 2019. Contains 329812 sequences. (Running on oeis4.)