Lehmer mean

Let $p$ be a real number. Lehmer mean of the positive numbers $a_{1},\,\ldots,\,a_{n}$ is defined as

\displaystyle L_{p}(a_{1},\,\ldots,\,a_{n})\;:=\;\frac{a_{1}^{p}+\ldots+a_{n}^% {p}}{a_{1}^{p-1}+\ldots+a_{n}^{p-1}}.

(1)

This definition fulfils both requirements set for means (http://planetmath.org/Mean3). In the case of Lehmer mean of two positive numbers $a$ and $b$ we see for $a\leqq b$ that

a\;=\;\frac{a^{p}\!+\!ab^{p-1}}{a^{p-1}\!+\!b^{p-1}}\;\leqq\;\frac{a^{p}\!+\!b% ^{p}}{a^{p-1}\!+\!b^{p-1}}\;\leqq\;\frac{a^{p-1}b\!+\!b^{p}}{a^{p-1}\!+\!b^{p-% 1}}\;=\;b.

The Lehmer mean of certain numbers is the greater the greater is the parametre $p$ , i.e.

L_{p}(a_{1},\,\ldots,\,a_{n})\;\geqq\;L_{q}(a_{1},\,\ldots,\,a_{n})\quad% \forall\;p\;>\;q.

This turns out from the nonnegativeness of the partial derivative of $L_{p}$ with respect to $p$ ; in the case $n=2$ it writes

\frac{\partial L_{p}}{\partial p}\;=\;\frac{a^{p-1}b^{p-1}(a\!-\!b)(\ln{a}-\ln% {b})}{(a^{p-1}\!+\!b^{p-1})^{2}}\;\geqq\;0.

Thus in the below list containing special cases of Lehmer mean, the is the least and the contraharmonic the greatest (cf. the comparison of Pythagorean means).

E.g. for two arguments $a$ and $b$ , one has

•

$\displaystyle L_{0}(a,\,b)\,=\,\frac{2ab}{a\!+\!b}$ , harmonic mean,
•

$\displaystyle L_{1/2}(a,\,b)\,=\,\sqrt{ab}$ , geometric mean,
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$\displaystyle L_{1}(a,\,b)\,=\,\frac{a\!+\!b}{2}$ , arithmetic mean,
•

$\displaystyle L_{2}(a,\,b)\,=\,\frac{a^{2}\!+\!b^{2}}{a\!+\!b}$ , contraharmonic mean.

Note. The least (http://planetmath.org/LeastNumber) and the greatest of the numbers (http://planetmath.org/GreatestNumber) $a_{1},\,\ldots,\,a_{n}$ may be regarded as borderline cases of the Lehmer mean, since

\lim_{p\to-\infty}L_{p}(a_{1},\,\ldots,\,a_{n})\;=\;\min\{a_{1},\,\ldots,\,a_{% n}\},\quad\lim_{p\to+\infty}L_{p}(a_{1},\,\ldots,\,a_{n})\;=\;\max\{a_{1},\,% \ldots,\,a_{n}\}.

For proving these equations, suppose that there are exactly $k$ greatest (resp. least) ones among the numbers and that those are $a_{1}=\ldots=a_{k}$ . Then we can write

L_{p}(a_{1},\,\ldots,\,a_{n})\;=\;\frac{a_{1}^{p}\left[k+\!\left(\frac{a_{k+1}% }{a_{1}}\right)^{p}\!+\ldots+\!\left(\frac{a_{n}}{a_{1}}\right)^{p}\right]}{a_% {1}^{p-1}\left[k+\!\left(\frac{a_{k+1}}{a_{1}}\right)^{p-1}\!+\ldots+\!\left(% \frac{a_{n}}{a_{1}}\right)^{p-1}\right]}.

Letting $p\to+\infty$ (resp. $p\to-\infty$ ), this equation yields

L_{p}(a_{1},\,\ldots,\,a_{n})\;\longrightarrow\;a_{1}.

Title	Lehmer mean
Canonical name	LehmerMean
Date of creation	2013-03-22 19:02:06
Last modified on	2013-03-22 19:02:06
Owner	pahio (2872)
Last modified by	pahio (2872)
Numerical id	11
Author	pahio (2872)
Entry type	Definition
Classification	msc 62-07
Classification	msc 11-00
Related topic	OrderOfSixMeans
Related topic	LeastAndGreatestNumber
Related topic	MinimalAndMaximalNumber