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Search: id:A104900
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| A104900 |
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Numbers n such that sigma(n)=6*phi(n). |
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+0
7
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| 6, 70, 616, 1240, 2090, 8932, 17980, 19780, 20320, 26980, 29512, 43180, 49742, 51688, 58058, 79000, 100130, 116870, 128570, 175370, 176715, 201376, 208280, 221536, 275770, 280670, 282680, 302176, 373065, 427924, 435435, 470764, 483616
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OFFSET
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1,1
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COMMENT
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If p>2 & 2^p-1 is prime (Merssene prime) then 5*2^(p-2)*(2^p-1) is in the sequence. So 5*2^(A000043-2)*(2^A000043-1) is a subsequence of this sequence.
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EXAMPLE
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p>2, q=2^p-1(q is prime); m=5*2^(p-2)*q so sigma(m)=6*(2^(p-1)-1)*2^p
=6*phi(m) hence m is in the sequence. sigm(79000)=187200=6*31200
=6*phi(79000) so 79000 is in the sequence but 79000 isn't of the
form 5*2^(p-2)*(2^p-1).
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MATHEMATICA
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Do[If[DivisorSigma[1, m] == 6*EulerPhi[m], Print[m]], {m, 1000000}]
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CROSSREFS
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Cf. A000043, A062699, A068390, A104901.
Sequence in context: A003362 A098639 A048708 this_sequence A001448 A024489 A036361
Adjacent sequences: A104897 A104898 A104899 this_sequence A104901 A104902 A104903
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KEYWORD
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easy,nonn
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AUTHOR
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Farideh Firoozbakht (f.firoozbakht(AT)math.ui.ac.ir), Apr 01 2005
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