OFFSET
1,1
COMMENTS
Primes trivially satisfy the defining condition.
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000
FORMULA
It seems that a(n) is asymptotic to c*n^2, 2<c<2.5 and that a(n)>2*n^2. - Benoit Cloitre, Sep 17 2002
EXAMPLE
sigma(15)-phi(15) = 24-8 = 16 divides sigma(15)-phi(15)=24+8 = 32, so 15 is a term of the sequence.
MATHEMATICA
f[n_] := Module[{a = DivisorSigma[1, n], b = EulerPhi[n]}, Mod[(a + b), (a - b)] == 0]; Select[Range[2, 10^4], (f[ # ] && ! PrimeQ[ # ]) &]
CROSSREFS
KEYWORD
nonn
AUTHOR
Joseph L. Pe, Feb 13 2002
STATUS
approved