OFFSET
1,2
COMMENTS
Numbers k such that sigma_0(k) divides phi(k) divides sigma_1(k).
REFERENCES
David Wells, Curious and interesting numbers, Penguin Books, p. 130.
LINKS
Donovan Johnson, Table of n, a(n) for n = 1..10000
EXAMPLE
210 has 16 divisors, which divides phi(210) = 48, which in turn divides sigma(210) = 576, so 210 is a term of the sequence.
MATHEMATICA
q[n_] := And @@ IntegerQ /@ Ratios @ {DivisorSigma[0, n], EulerPhi[n], DivisorSigma[1, n]}; Select[Range[6000], q] (* Amiram Eldar, Apr 13 2024 *)
PROG
(PARI) for(n=1, 1e3, if(sigma(n)%eulerphi(n)==0, if(sigma(n)%numdiv(n)==0, if(eulerphi(n)%numdiv(n)==0, print1(n, ", "))))) \\ Felix Fröhlich, Aug 08 2014
CROSSREFS
KEYWORD
nonn
AUTHOR
EXTENSIONS
Wells incorrectly has 52 instead of 56.
STATUS
approved