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Numbers n such that phi(n) and sigma(n) have the same number of divisors.
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%I #7 Sep 29 2019 02:51:00

%S 1,4,7,18,19,21,39,41,45,72,77,99,103,124,133,137,143,147,153,171,172,

%T 194,199,215,231,242,247,261,268,275,287,288,307,309,320,327,335,336,

%U 349,350,363,364,366,372,399,434,436,477,490,491,551,575,597,605,639

%N Numbers n such that phi(n) and sigma(n) have the same number of divisors.

%H Amiram Eldar, <a href="/A080650/b080650.txt">Table of n, a(n) for n = 1..10000</a>

%t Select[Range[10^3], DivisorSigma[0, DivisorSigma[1, # ]] == DivisorSigma[0, EulerPhi[ # ]] &]

%Y Cf. A000005, A000010, A000203.

%K nonn

%O 1,2

%A _Joseph L. Pe_, Feb 28 2003