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Search: id:A055234
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| A055234 |
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Smallest x such that sigma(x) = n*phi(x), or -1 if no such x exists. |
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+0
9
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| 1, 3, 2, 14, 56, 6, 12, 42, 30, 168, 2580, 210, 630, 420, 840, 20790, 416640, 9240, 291060, 83160, 120120, 5165160, 1719277560, 43825320, 26860680, 277560360, 1304863560, 569729160, 587133466920, 16522145640, 33044291280
(list; graph; listen)
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OFFSET
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1,2
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COMMENT
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Conjecture: For each n, a(n) > 0. - Farideh Firoozbakht (mymontain(AT)yahoo.com), Sep 12 2004
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FORMULA
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a(n) = Min{x : A000203(x)/A000010(x) = n} = Min{x : A023897(x) = n}
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EXAMPLE
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sigma(14) = 24 = 4*phi(14), so a(4) = 14.
n = 21: a(21) = 120120 = 2.2.2.3.5.7.11.13, sigma(120120) = 483840 = n*phi(120120), phi(120120) = 23040.
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MATHEMATICA
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a[n_]:=(For[m=1, DivisorSigma[1, m]!=n EulerPhi[m], m++ ]; m); Do[Print[a[n]], {n, 31}] [From Farideh Firoozbakht (mymontain(AT)yahoo.com), Oct 31 2008]
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CROSSREFS
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Cf. A000203, A000010, A020492, A023897, A088830.
Adjacent sequences: A055231 A055232 A055233 this_sequence A055235 A055236 A055237
Sequence in context: A064536 A163355 A145747 this_sequence A086485 A068310 A033314
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KEYWORD
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nonn
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AUTHOR
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Jud McCranie (j.mccranie(AT)comcast.net), Jun 21 2000
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EXTENSIONS
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More terms from Farideh Firoozbakht (mymontain(AT)yahoo.com), Sep 12 2004
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