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A271633
Numbers k such that sigma(phi(k)) - phi(k) = phi(sigma(k)), where phi(k) is the Euler totient function of k and sigma(k) is the sum of the divisors of k.
1
21, 350, 366, 532, 702, 1072, 5264, 7128, 23604, 24102, 30222, 30636, 32142, 32274, 34350, 47338, 70722, 78530, 113550, 137214, 197316, 235624, 292206, 357490, 367704, 398346, 406596, 453096, 453264, 464820, 479880, 485460, 504966, 509124, 512430, 519870, 539220
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