OFFSET
1,1
COMMENTS
There are no terms between 449 and 10^5. Are there any more terms? Are there infinitely many?
No more terms below 10^9. - Amiram Eldar, Sep 26 2019
LINKS
J. Pe, On a Generalization of Perfect Numbers, J. Rec. Math., 31(3) (2002-2003), 168-172.
EXAMPLE
Let f(n) = EulerPhi(sigma(EulerPhi(n))). Proper divisors of 28 = {1, 2, 4, 7, 14}; the sum of their f-values = 1+1+2+4+4 = 12 = f(28); hence 28 belongs to the sequence.
MATHEMATICA
f[x_] := EulerPhi[DivisorSigma[1, EulerPhi[x]]]; Select[ Range[ 1, 10^5], 2 * f[ # ] == Apply[ Plus, Map[ f, Divisors[ # ] ] ] & ]
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Joseph L. Pe, Dec 18 2001
EXTENSIONS
a(6) from Amiram Eldar, Sep 26 2019
STATUS
approved