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%I #16 Dec 01 2020 02:54:10
%S 1,2,7,9,14,18,29,58,213,271,284,426,542,673,731,791,833,1011,1015,
%T 1017,1131,1305,1346,1348,1376,1462,1508,1568,1582,1624,1666,1720,
%U 1960,2022,2030,2034,2064,2088,2262,2352,2436,2580,2610,2940,2971,5942,7775
%N Numbers k such that phi(k) is a harmonic number.
%C Recall that k is harmonic if the harmonic mean of its divisors is an integer, i.e. if k * tau(k) / sigma(k) is an integer (Tattersall, p. 147).
%D James J. Tattersall, Elementary Number Theory in Nine Chapters, Cambridge Univ. Press, 2001.
%H Amiram Eldar, <a href="/A074244/b074244.txt">Table of n, a(n) for n = 1..10000</a> (terms 1..1000 from Donovan Johnson)
%e phi(14) = 6 and 6 * tau(6) / sigma(6) = 6 * 4 / 12 = 2, an integer, so 14 is a term of the sequence.
%t isHarmonic[n_] := IntegerQ[n*DivisorSigma[0, n] / DivisorSigma[1, n]]; Select[Range[10^4], isHarmonic[EulerPhi[ # ]] &]
%Y Cf. A000010, A001599.
%K nonn
%O 1,2
%A _Joseph L. Pe_, Sep 19 2002
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