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Numbers k such that sigma(phi(sigma(k))) = sigma(k).
1

%I #19 Dec 19 2024 21:16:42

%S 1,2,8,12,128,240,350,366,368,575,671,720,743,1098,2417,6912,18967,

%T 19399,32768,142560,173340,179010,184032,188892,203310,209628,225342,

%U 228582,232160,244782,255460,271072,274422,279045,282285,287810,304910,307612

%N Numbers k such that sigma(phi(sigma(k))) = sigma(k).

%H Harry J. Smith and Donovan Johnson, <a href="/A066471/b066471.txt">Table of n, a(n) for n = 1..500</a> (first 250 terms from Harry J. Smith)

%t Select[Range[1, 10^6], DivisorSigma[1, EulerPhi[DivisorSigma[1, # ]]] == DivisorSigma[1, # ] &]

%o (PARI) isok(k) = { sigma(eulerphi(sigma(k))) == sigma(k) } \\ _Harry J. Smith_, Feb 17 2010

%K nonn

%O 1,2

%A _Joseph L. Pe_, Jan 02 2002