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

${\displaystyle \frac{m-x}{y-m}}={\displaystyle \frac{y}{x}}.$ (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:

$$\frac{m-x}{y-m}=\frac{m}{m}\mathit{\hspace{1em}\hspace{1em}}(\text{arithmetic mean }m)$$

$$\frac{m-x}{y-m}=\frac{m}{y}\mathit{\hspace{1em}\hspace{1em}}(\text{geometric mean }m)$$

$$\frac{m-x}{y-m}=\frac{x}{y}\mathit{\hspace{1em}\hspace{1em}}(\text{harmonic mean }m)$$

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

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

$$\frac{c+h}{2}=a\mathit{\hspace{1em}}\text{and}\mathit{\hspace{1em}}\sqrt{ah}=g.$$

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:

$$\frac{2^2+6^2}{2+6}=\frac{40}{8}=\mathrm{\hspace{0.33em}5},\frac{3^2+6^2}{3+6}=\frac{45}{9}=\mathrm{\hspace{0.33em}5}$$

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:

$$c(x_1,\mathrm{\dots },x_n):=\frac{x_1^2+\mathrm{\dots }+x_n^2}{x_1+\mathrm{\dots }+x_n}$$

There is also a more general Lehmer mean:

$$c^m(x_1,\mathrm{\dots },x_n):=\frac{x_1^{m+1}+\mathrm{\dots }+x_n^{m+1}}{x_1^m+\mathrm{\dots }+x_n^m}$$

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