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A164648
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Numbers n such that sigma(n)/phi(n) = 25/16.
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4
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40859, 48505, 54385, 121771, 156125, 565607, 1154419, 1219933, 1294363, 2448397, 3590461, 9710975, 16067363, 16069573, 17984515, 19013455, 21341755, 25804115, 26515223, 27656155, 29655415, 30372605
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,1
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COMMENTS
| 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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MATHEMATICA
| Select[Range[1000000, 2000000], DivisorSigma[1, #]/EulerPhi[#] == 25/16 &] (* Carl Najafi, Aug 16 2011 *)
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PROG
| (PARI) for( n=1, 1e7, sigma(n)==25/16*eulerphi(n) && print1(n", "))
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CROSSREFS
| Cf. A000010 (=phi), A000203 (=sigma), A068390 (sigma/phi=4), A163667 (sigma/phi=9), A164646 (sigma/phi=9/4).
Sequence in context: A090060 A097238 A046180 * A097479 A133863 A033532
Adjacent sequences: A164645 A164646 A164647 * A164649 A164650 A164651
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KEYWORD
| more,nonn
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AUTHOR
| M. F. Hasler (www.univ-ag.fr/~mhasler), Aug 22 2009
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EXTENSIONS
| More terms from Carl Najafi (carlnajafi(AT)gmail.com), Aug 16 2011
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