OFFSET
1,1
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..215 (terms below 10^9)
EXAMPLE
sigma(phi(21)) - phi(21) = 28 - 12 = 16 = phi(sigma(21)).
MAPLE
with(numtheory): P:= proc(q) local n; for n from 1 to q do
if sigma(phi(n))-phi(n)=phi(sigma(n)) then print(n); fi;
od; end: P(10^6);
MATHEMATICA
Select[Range[10^6], DivisorSigma[1, #] - # &@ EulerPhi@ # == EulerPhi@ DivisorSigma[1, #] &] (* Michael De Vlieger, Apr 21 2016 *)
PROG
(PARI) isok(k) = my(x=eulerphi(k)); sigma(x) - x == eulerphi(sigma(k)); \\ Michel Marcus, Jul 13 2019
CROSSREFS
KEYWORD
nonn
AUTHOR
Paolo P. Lava, Apr 19 2016
STATUS
approved