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Search: id:A164648
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| A164648 |
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Numbers n such that sigma(n)/phi(n) = 25/16. |
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5
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| 40859, 48505, 54385, 121771, 156125, 565607, 1154419, 1219933, 1294363, 2448397
(list; graph; listen)
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OFFSET
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1,1
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COMMENT
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A subsequence of A011257.
If 5^{k+1}-1 = d*D such that p = 2*5^{k+1}*(d+1)-1 and q = 2*(5^{k+1}+D)-1 are distinct primes, then n = 5^k*p*q is a term of this sequence.
The same theorem holds for sequences of numbers such that sigma/phi=b^2/(b-1)^2 with other primes b (here b=5), cf. A164646.
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PROGRAM
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(PARI) for( n=1, 1e7, sigma(n)==25/16*eulerphi(n) && print1(n", "))
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CROSSREFS
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Cf. A000010 (=phi), A000203 (=sigma), A068390 (sigma/phi=4), A163667 (sigma/phi=9), A164646 (sigma/phi=9/4).
Sequence in context: A090060 A097238 A046180 this_sequence A097479 A133863 A033532
Adjacent sequences: A164645 A164646 A164647 this_sequence A164649 A164650 A164651
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KEYWORD
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more,nonn
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AUTHOR
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M. F. Hasler (mhasler(AT)univ-ag.fr), Aug 22 2009
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