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A033632
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Numbers n such that sigma(phi(n))=phi(sigma(n)).
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19
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1, 9, 225, 242, 516, 729, 3872, 13932, 14406, 17672, 18225, 20124, 21780, 29262, 29616, 45996, 65025, 76832, 92778, 95916, 106092, 106308, 114630, 114930, 121872, 125652, 140130, 140625, 145794, 149124, 160986, 179562, 185100
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,2
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COMMENTS
| The largest term of this sequence that I found is 3^9550. Also, if (1/2)*(3^(n+1)-1) is prime (n+1 is a term of A028491) then 3^n is in the sequence, namely sigma(phi(3^n) = phi(sigma(3^n)) (the proof is easy). - Farideh Firoozbakht (mymontain(AT)yahoo.com), Feb 09 2005
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LINKS
| T. D. Noe, Table of n, a(n) for n=1..200
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MATHEMATICA
| Select[ Range[ 10^6 ], DivisorSigma[ 1, EulerPhi[ # ] ] == EulerPhi[ DivisorSigma[ 1, # ] ] & ]
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CROSSREFS
| Cf. A000203, A000010.
Cf. A028491.
Cf. A078148
Sequence in context: A205578 A079913 A205568 * A110260 A036896 A120319
Adjacent sequences: A033629 A033630 A033631 * A033633 A033634 A033635
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KEYWORD
| nonn,easy,nice
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AUTHOR
| Jud McCranie (JudMcCranie(AT)ugaalum.uga.edu)
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