Search: id:A005279
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%I A005279 M4093
%S A005279 6,12,15,18,20,24,28,30,35,36,40,42,45,48,54,56,60,63,66,70,72,75,77,78,
%T A005279 80,84,88,90,91,96,99,100,102,104,105,108,110,112,114,117,120,126,130,
%U A005279 132,135,138,140,143,144,150,153,154,156,160,162,165,168,170,174,175,176
%N A005279 Numbers having divisors d,e with d < e < 2d.
%C A005279 Arithmetic and harmonic means of A046793(n) and a(n) both integers.
%C A005279 n is in this sequence iff n is a multiple of some term in A020886.
%C A005279 a(n) is also a positive integer v for which there exists a smaller positive 
               integer u such that the contraharmonic mean (uu+vv)/(u+v) is an integer 
               c (in fact, there are two distinct values u giving with v the same 
               c). [From Pahikkala Jussi (pahio(AT)wakkanet.fi), Dec 14 2008]
%D A005279 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, 
               Academic Press, 1995 (includes this sequence).
%D A005279 R. K. Guy, Unsolved Problems in Number Theory, E3.
%H A005279 Planet Math., <a href="http://planetmath.org/encyclopedia/IntegerContraharmonicMeans.html">
               Proposition 4</a>
%H A005279 Planet Math., <a href="http://planetmath.org/encyclopedia/ContraharmonicProportion.html">
               Definition</a>
%p A005279 isA005279 := proc(n) local divs,d,e ; divs := numtheory[divisors](n) 
               ; for d from 1 to nops(divs)-1 do for e from d+1 to nops(divs) do 
               if divs[e] < 2*divs[d] then RETURN(true) ; fi ; od: od: RETURN(false) 
               : end; for n from 3 to 300 do if isA005279(n) then printf("%d,",n) 
               ; fi ; od : - R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Jun 08 
               2006
%Y A005279 Cf. A010814.
%Y A005279 Sequence in context: A114304 A107487 A092671 this_sequence A129512 A116359 
               A141698
%Y A005279 Adjacent sequences: A005276 A005277 A005278 this_sequence A005280 A005281 
               A005282
%K A005279 nonn
%O A005279 1,1
%A A005279 N. J. A. Sloane (njas(AT)research.att.com), Robert G. Wilson v (rgwv(AT)rgwv.com)

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