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A068560 Numbers k such that phi(k) = tau(k)^2. 5
1, 5, 34, 63, 76, 128, 136, 170, 315, 364, 380, 444, 640, 680, 972, 1820, 1824, 1836, 2142, 2220, 4788, 4860, 6000, 8064, 8568, 8736, 9120, 9180, 10710, 23940, 40320, 42840, 43680 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,2
COMMENTS
No more terms through 10^9. - Ryan Propper, Jun 09 2006
This sequence is finite because phi(k) >= sqrt(k) for all k >= 6, and for any e > 0, tau(k) < k^e for k large enough. Choosing e=1/4 gives tau(k)^2 < sqrt(k) <= phi(k). It remains unknown, however, if this sequence is full. - Nathaniel Johnston, Apr 28 2011
LINKS
MATHEMATICA
Do[If[EulerPhi[n] == DivisorSigma[0, n]^2, Print[n]], {n, 10^5}] (* Ryan Propper, Jun 09 2006 *)
Select[Range[10^5], EulerPhi[#] == DivisorSigma[0, #]^2 &] (* Alonso del Arte, Aug 25 2011 *)
CROSSREFS
Cf. A039770.
Sequence in context: A213063 A268281 A223137 * A039773 A289947 A272955
KEYWORD
easy,fini,nonn
AUTHOR
Benoit Cloitre, Mar 25 2002
EXTENSIONS
More terms from Ryan Propper, Jun 09 2006
STATUS
approved

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Last modified March 19 06:47 EDT 2024. Contains 370953 sequences. (Running on oeis4.)