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A020488
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Numbers n such that tau(n) (or sigma_0(n)) = phi(n).
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19
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OFFSET
| 1,2
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COMMENTS
| Numbers satisfying A000005(n)=A000010(n)
This sequence is complete because tau(n) < n^(2/3) for all n except a few small numbers, whereas phi(n) > n/(exp(gamma) * log(log(n)) + 3/(log(log(n))) for n > 2. log(log(n)) grows slowly, so phi(n) > tau(n) for all n > some relatively small constant. [Jud McCranie, Jun 17 2005]
Subset of A112587. - Reinhard Zumkeller, Sep 14 2005
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MAPLE
| select(k->tau(k)=phi(k), [$1..1000]); # Peter Luschny, Aug 26 2011
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MATHEMATICA
| k=1; s=Select[ Range[ 1, 100000 ], Equal[ Sign[ DivisorSigma[ k-1, # ]-EulerPhi[ # ]^k ], 0 ]& ]
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CROSSREFS
| Cf. A064374-A064377, A000005, A000010.
Cf. A112954, A062516, A063469, A063470.
Sequence in context: A143144 * A064435 A079541 A131725 A032914 A088072
Adjacent sequences: A020485 A020486 A020487 * A020489 A020490 A020491
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KEYWORD
| nonn,fini,full
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AUTHOR
| David W. Wilson (davidwwilson(AT)comcast.net)
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