OFFSET
1,1
COMMENTS
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..1000
EXAMPLE
40 is in the sequence because phi(40)= 16, phi(16) = 8, sopf(40) = 7 and sigma(7) = 8;
171 is in the sequence because phi(171) = 108, phi(108) = 36, sopf(171) = 22 and sigma(22) = 36.
MAPLE
with(numtheory) :
A008472 := proc(n)
add(p, p = factorset(n):
end proc:
isA173337 := proc(n)
phi(phi(n)) = sigma(A008472(n)) ;
end proc:
for n from 1 do
if isA173337(n) then printf("%d, ", n) ; fi;
end do: # R. J. Mathar, Jul 06 2012
MATHEMATICA
sopf[n_] := Plus @@ (First@# & /@ FactorInteger[n]); Select[Range[2, 13000], EulerPhi[EulerPhi[#]] == DivisorSigma[1, sopf[#]] &] (* Amiram Eldar, Jul 09 2019 *)
Select[Range[2, 15000], DivisorSigma[1, Total[FactorInteger[#][[All, 1]]]] == EulerPhi[ EulerPhi[#]]&] (* Harvey P. Dale, Apr 05 2020 *)
PROG
(PARI) isok(n) = (n>1) && eulerphi(eulerphi(n)) == sigma(vecsum(factor(n)[, 1])); \\ Michel Marcus, Jul 10 2019
CROSSREFS
KEYWORD
nonn
AUTHOR
Michel Lagneau, Feb 16 2010
EXTENSIONS
Definition clarified by N. J. A. Sloane, Apr 05 2020
STATUS
approved