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Search: id:A136547
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| A136547 |
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Numbers n such that sigma(n)=5*phi(n). |
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+0
1
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| 56, 190, 812, 1672, 4522, 5278, 16065, 24244, 25070, 33915, 39585, 56252, 80104, 93496, 102856, 107156, 140296, 157586, 220616, 224536, 316274, 317205, 365638, 389732, 423045, 479655, 546592, 559845, 596666, 601312, 696514, 731962, 1123605
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OFFSET
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1,1
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COMMENT
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If p>2 and 2^p-1 is prime (a Mersenne prime) then 377*2^(p-2)*(2^p-1) is in the sequence (the proof is easy). So for n>1 377*2^(A000043(n)-2)*(2^A000043(n)-1) is in the sequence.
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EXAMPLE
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sigma(56)=120=5*24=5*phi(56) so 56 is in the sequence.
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MATHEMATICA
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Do[If[DivisorSigma[1, m]==5*EulerPhi[m], Print[m]], {m, 1500000}]
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CROSSREFS
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Cf. A000043, A062699, A104900, A104901, A104902.
Sequence in context: A044769 A154034 A115620 this_sequence A158481 A158487 A110554
Adjacent sequences: A136544 A136545 A136546 this_sequence A136548 A136549 A136550
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KEYWORD
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nonn
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AUTHOR
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Farideh Firoozbakht (mymontain(AT)yahoo.com), Jan 29 2008, Jan 30 2008
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