# contraharmonic proportion

Three positive numbers  $x$, $m$, $y$  are in contraharmonic proportion, if the ratio of the difference of the second and the first number to the difference of the third and the second number is equal the ratio of the third and the first number, i.e. if

 $\displaystyle\frac{m\!-\!x}{y\!-\!m}\;=\;\frac{y}{x}.$ (1)

The middle number $m$ is then called the contraharmonic mean (sometimes antiharmonic mean) of the first and the last number.

The contraharmonic proportion has very probably been known in the proportion doctrine of the Pythagoreans, since they have in a manner to (1) described the classical Babylonian means:

 $\frac{m\!-\!x}{y\!-\!m}\;=\;\frac{m}{m}\qquad(\mbox{arithmetic mean }m)$
 $\frac{m\!-\!x}{y\!-\!m}\;=\;\frac{m}{y}\qquad(\mbox{geometric mean }m)$
 $\frac{m\!-\!x}{y\!-\!m}\;=\,\frac{x}{y}\qquad(\mbox{harmonic mean }m)$

The contraharmonic mean $m$ is between $x$ and $y$. Indeed, if we solve it from (1), we get

 $\displaystyle m\;=\;\frac{x^{2}\!+\!y^{2}}{x\!+\!y},$ (2)

and if we assume that  $x\leqq y$, we see that

 $x\;=\;\frac{x^{2}\!+\!xy}{x\!+\!y}\;\leqq\;\frac{x^{2}\!+\!y^{2}}{x\!+\!y}\;% \leqq\;\frac{xy\!+\!y^{2}}{x\!+\!y}\;=\;y.$

The contraharmonic mean $c$ is the greatest of all the mentioned means,

 $x\;\leqq\;h\;\leqq\;g\;\leqq\;a\;\leqq\;c\;\leqq\;y,$

where $a$ is the arithmetic mean, $g$ the geometric mean and $h$ the harmonic mean. It is easy to see that

 $\frac{c\!+\!h}{2}\;=\;a\quad\mbox{and}\quad\sqrt{ah}\;=\;g.$

Example.  The integer 5 is the contraharmonic mean of 2 and 6, as well as of 3 and 6, i.e.   2, 5, 6,  are in contraharmonic proportion, similarly are  3, 5, 6:

 $\frac{2^{2}\!+\!6^{2}}{2\!+\!6}\;=\;\frac{40}{8}\;=\;5,\qquad\frac{3^{2}\!+\!6% ^{2}}{3\!+\!6}\;=\;\frac{45}{9}\;=\;5$

Note 1.  The graph of (2) is a quadratic cone surface  $x^{2}\!+\!y^{2}\!-xz\!-\!yz=0$, as one may infer of its level curves

 $\left(x-\frac{c}{2}\right)^{2}+\left(y-\frac{c}{2}\right)^{2}\;=\;\frac{c^{2}}% {2}$

which are circles.

Note 2.  Generalising (2) one defines the contraharmonic mean of several positive numbers:

 $c(x_{1},\,\ldots,\,x_{n}):=\frac{x_{1}^{2}\!+\ldots+\!x_{n}^{2}}{x_{1}\!+% \ldots+\!x_{n}}$

There is also a more general Lehmer mean:

 $c^{m}(x_{1},\,\ldots,\,x_{n}):=\frac{x_{1}^{m+1}\!+\ldots+\!x_{n}^{m+1}}{x_{1}% ^{m}\!+\ldots+\!x_{n}^{m}}$

## References

• 1 Diderot & d’Alembert: Encyclopédie. Paris (1751–1777).  (Electronic version http://portail.atilf.fr/encyclopedie/here).
• 2 Horst Hischer: “http://hischer.de/uds/forsch/preprints/hischer/Preprint98.pdfViertausend Jahre Mittelwertbildung”.  — mathematica didactica 25 (2002). See also http://www.math.uni-sb.de/PREPRINTS/preprint126.pdfthis.
• 3 J. Pahikkala: “On contraharmonic mean and Pythagorean triples”.  – Elemente der Mathematik 65:2 (2010).
 Title contraharmonic proportion Canonical name ContraharmonicProportion Date of creation 2015-09-06 19:48:08 Last modified on 2015-09-06 19:48:08 Owner pahio (2872) Last modified by pahio (2872) Numerical id 24 Author pahio (2872) Entry type Definition Classification msc 26E60 Classification msc 11-00 Classification msc 01A17 Classification msc 01A20 Related topic ProportionEquation Related topic Mean3 Related topic PythagoreanHypotenusesAsContraharmonicMeans Related topic HarmonicMean Related topic ConstructionOfContraharmonicMeanOfTwoSegments Related topic ContrageometricProportion Defines contraharmonic mean Defines antiharmonic mean