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Search: id:A066950
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| A066950 |
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Numbers n such that phi(sigma(n)) + sigma(phi(n)) = 2n. |
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+0
1
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| 1, 11, 18, 75, 172, 372, 482, 732, 3276, 7965, 13068, 46476, 103488, 252240, 727956, 1795605, 6030528, 8769600, 15436092, 56747712, 224518995, 297780084, 317961000, 855489144, 2691137721
(list; graph; listen)
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OFFSET
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1,2
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COMMENT
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a(26) > 3*10^9. [From Donovan Johnson (donovan.johnson(AT)yahoo.com), Sep 16 2009]
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EXAMPLE
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phi(sigma(11)) + sigma(phi(11)) = phi(12) + sigma(10) = 4 + 18 = 2*11, so 11 is in the sequence.
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MATHEMATICA
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f[n_] := EulerPhi[DivisorSigma[1, n]]+DivisorSigma[1, EulerPhi[n]]; Select[Range[1, 10^5], f[ # ]==2*#&]
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CROSSREFS
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Cf. A000010, A000203.
Sequence in context: A003334 A037006 A168433 this_sequence A162011 A123248 A111477
Adjacent sequences: A066947 A066948 A066949 this_sequence A066951 A066952 A066953
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KEYWORD
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more,nonn
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AUTHOR
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Joseph L. Pe (joseph_l_pe(AT)hotmail.com), Jan 25 2002
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EXTENSIONS
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Edited by Dean Hickerson (dean.hickerson(AT)yahoo.com), Jan 26 2002
a(20)-a(25) from Donovan Johnson (donovan.johnson(AT)yahoo.com), Sep 16 2009
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