OFFSET
1,2
REFERENCES
M. Abramowitz and I. A. Stegun, eds., Handbook of Mathematical Functions, National Bureau of Standards Applied Math. Series 55, 1964 (and various reprintings), p. 840.
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000
W. Sierpinski, Number Of Divisors And Their Sum
Wikipedia, Euler's totient function
EXAMPLE
For n=4, phi(tau(4)) = phi(3)=2 equals tau(rad(4)) = tau(2) = 2, so n=4 is in the sequence.
For n=108, phi(tau(108) ) = phi(12) = 4 equals tau(rad(108)) = tau(6) = 4, so n =108 is in the sequence.
MAPLE
with(numtheory): for n from 1 to 500 do :t1:= ifactors(n)[2] : t2 :=mul(t1[i][1], i=1..nops(t1)):if phi(tau(n)) = tau(t2) then print (n): else fi:od:
MATHEMATICA
rad[n_] := Times @@ (First@# & /@ FactorInteger[n]); Select[Range[360], EulerPhi[ DivisorSigma[0, #] ] == DivisorSigma[0, rad[#]] &] (* Amiram Eldar, Jul 09 2019 *)
PROG
(Magma) [ k:k in [1..360]| EulerPhi(#Divisors(k)) eq #Divisors(&*PrimeDivisors(k)) ]; // Marius A. Burtea, Jul 09 2019
CROSSREFS
KEYWORD
nonn
AUTHOR
Michel Lagneau, Feb 23 2010
STATUS
approved