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Inequality of arithmetic and geometric means

In mathematics, the inequality of arithmetic and geometric means, or more briefly the AM-GM inequality, states that the arithmetic mean of a list of non-negative real numbers is greater than or equal to the geometric mean of the same list; and further, that the two means are equal if and only if every number in the list is the same.

Contents

1 Background

2 The inequality

3 Proof

3.1 The case where all the terms are equal
3.2 The case where not all the terms are equal

3.2.1 The subcase where n = 2
3.2.2 The subcase where n = 2k
3.2.3 The subcase where n < 2k

4 Example application

5 Generalization

Background

The arithmetic mean, or less precisely the average, of a list of n numbers x₁, x₂, . . ., x_n is the sum of the numbers divided by n:

$\frac{x_1 + x_2 + \cdots + x_n}{n}.$

The geometric mean is similar, except that it is only defined for a list of nonnegative real numbers, and uses multiplication and involution in place of addition and multiplication:

$\sqrt[n]{x_1 \cdot x_2 \cdots x_n}.$

This is equivalent to the antilogarithm of the arithmetic mean of the list of logarithms of the numbers:

$\exp \left( \frac{\ln {x_1} + \ln {x_2} + \cdots + \ln {x_n}}{n} \right).$

The inequality

Restating the inequality using mathematical notation, we have that for any list of n nonnegative real numbers x₁, x₂, . . ., x_n,

$\frac{x_1 + x_2 + \cdots + x_n}{n} \geq \sqrt[n]{x_1 \cdot x_2 \cdots x_n}$ ,

and that if and only if x₁ = x₂ = . . . = x_n,

$\frac{x_1 + x_2 + \cdots + x_n}{n} = \sqrt[n]{x_1 \cdot x_2 \cdots x_n}$ .

Proof

There are several ways to prove the AM-GM inequality; for example, it can be inferred from Jensen's inequality, using the concave function ln(x).[1] It can also be proven using the rearrangement inequality.

The following proof by cases, which was inspired by the explanation given at http://nrich.maths.org/askedNRICH/edited/1433.html, relies directly on well-known rules of arithmetic:

The case where all the terms are equal

If all the terms are equal:

$x_1 = x_2 = \cdots = x_n$

then their sum is nx₁, so their arithmetic mean is x₁; and their product is x₁ⁿ, so their geometric mean is x₁; therefore, the arithmetic mean and geometric mean are equal, as desired.

The case where not all the terms are equal

It remains to show that if not all the terms are equal, then the arithmetic mean is greater than the geometric mean. Clearly, this is only possible when n > 1.

This case is significantly more complex, and we divide it into subcases.

The subcase where n = 2

If n = 2, then we have two terms, x₁ and x₂, and since (by our assumption) not all terms are equal, we have:

$x_1\,\;$	$\ne x_2\,\;$
$x_1 - x_2\,\;$	$\ne 0\,\;$
$\left( x_1 - x_2 \right) ^2\,\;$	$> 0\,\;$
$x_1^2 - 2 x_1 x_2 + x_2^2\,\;$	$> 0\,\;$
$x_1^2 + 2 x_1 x_2 + x_2^2\,\;$	$> 4 x_1 x_2\,\;$
$\left( x_1 + x_2 \right) ^2\,\;$	$> 4 x_1 x_2\,\;$
$\left( \frac{x_1 + x_2}{2} \right)^2\,\;$	$> x_1 x_2\,\;$
$\frac{x_1 + x_2}{2}\,\;$	$> \sqrt{x_1 x_2}\,\;$

as desired.

The subcase where n = 2^k

Consider the case where n = 2^k, where k is a positive integer. We proceed by mathematical induction.

In the base case, k = 1, so n = 2. We have already shown that the inequality holds where n = 2, so we are done.

Now, suppose that for a given k > 1, we have already shown that the inequality holds for n = 2^k−1, and we wish to show that it holds for n = 2^k. To do so, we proceed as follows:

$\frac{x_1 + x_2 + \cdots + x_{2^k}}{2^k}$	$=\frac{\frac{x_1 + x_2 + \cdots + x_{2^{k-1}}}{2^{k-1}} + \frac{x_{2^{k-1} + 1} + x_{2^{k-1} + 2} + \cdots + x_{2^k}}{2^{k-1}}}{2}$

	$\ge \frac{\sqrt[2^{k-1}]{x_1 \cdot x_2 \cdots x_{2^{k-1}}} + \sqrt[2^{k-1}]{x_{2^{k-1} + 1} \cdot x_{2^{k-1} + 2} \cdots x_{2^k}}}{2}$

	$\ge \sqrt{\sqrt[2^{k-1}]{x_1 \cdot x_2 \cdots x_{2^{k-1}}} \cdot \sqrt[2^{k-1}]{x_{2^{k-1} + 1} \cdot x_{2^{k-1} + 2} \cdots x_{2^k}}}$

	$= \sqrt[2^k]{x_1 \cdot x_2 \cdots x_{2^k}}$

where in the first inequality, the two sides are only equal if both of the following are true:

$x_1 = x_2 = \cdots = x_{2^{k-1}}$

$x_{2^{k-1}+1} = x_{2^{k-1}+2} = \cdots = x_{2^k}$

(in which case the first arithmetic mean and first geometric mean are both equal to x₁, and similarly with the second arithmetic mean and second geometric mean); and in the second inequality, the two sides are only equal if the two geometric means are equal. Since not all 2^k numbers are equal, it's not possible for both inequalities to be equalities, so we know that:

$\frac{x_1 + x_2 + \cdots + x_{2^k}}{2^k} > \sqrt[2^k]{x_1 \cdot x_2 \cdots x_{2^k}}$

as desired.

The subcase where n < 2^k

If n is not a natural power of 2, then it is certainly less than some natural power of 2, since the sequence 2, 4, 8, . . ., 2^k, . . . is unbounded above. Therefore, without loss of generality, let m be some natural power of 2 that is greater than n.

So, if we have n terms, then let us denote their arithmetic mean by α, and expand our list of terms thus:

$x_{n+1} = x_{n+2} = \cdots = x_m = \alpha.$

We then have:

$\alpha\,\;$	$= \frac{x_1 + x_2 + \cdots + x_n}{n}$

	$= \frac{\frac{m}{n} \left( x_1 + x_2 + \cdots + x_n \right)}{m}$

	$= \frac{x_1 + x_2 + \cdots + x_n + \frac{m-n}{n} \left( x_1 + x_2 + \cdots + x_n \right)}{m}$

	$= \frac{x_1 + x_2 + \cdots + x_n + \left( m-n \right) \alpha}{m}$

	$= \frac{x_1 + x_2 + \cdots + x_n + x_{n+1} + \cdots + x_m}{m}$

	$> \sqrt[m]{x_1 \cdot x_2 \cdots x_n \cdot x_{n+1} \cdots x_m}$

	$= \sqrt[m]{x_1 \cdot x_2 \cdots x_n \cdot \alpha^{m-n}}$
so
$\alpha^m\,\;$	$> x_1 \cdot x_2 \cdots x_n \cdot \alpha^{m-n}$
$\alpha^n\,\;$	$> x_1 \cdot x_2 \cdots x_n$
$\alpha\,\;$	$> \sqrt[n]{x_1 \cdot x_2 \cdots x_n}$

as desired.

Example application

Consider the following function:

$f(x,y,z) = \frac{x}{y} + \sqrt{\frac{y}{z}} + \sqrt[3]{\frac{z}{x}}$

for x, y, and z all positive real numbers. Suppose we wish to find the minimum value of this function. Rewriting a bit, and applying the AM-GM inequality, we have:

$f(x,y,z)\,\;$	$= \frac{x}{y} + \frac{1}{2}\sqrt{\frac{y}{z}} + \frac{1}{2}\sqrt{\frac{y}{z}} + \frac{1}{3}\sqrt[3]{\frac{z}{x}} + \frac{1}{3}\sqrt[3]{\frac{z}{x}} + \frac{1}{3}\sqrt[3]{\frac{z}{x}}$

	$= 6 \times \frac{ \frac{x}{y} + \frac{1}{2} \sqrt{\frac{y}{z}} + \frac{1}{2} \sqrt{\frac{y}{z}} + \frac{1}{3} \sqrt[3]{\frac{z}{x}} + \frac{1}{3} \sqrt[3]{\frac{z}{x}} + \frac{1}{3} \sqrt[3]{\frac{z}{x}} }{6}$

	$\ge 6 \times \sqrt[6]{ \frac{x}{y} \cdot \frac{1}{2} \sqrt{\frac{y}{z}} \cdot \frac{1}{2} \sqrt{\frac{y}{z}} \cdot \frac{1}{3} \sqrt[3]{\frac{z}{x}} \cdot \frac{1}{3} \sqrt[3]{\frac{z}{x}} \cdot \frac{1}{3} \sqrt[3]{\frac{z}{x}} }$

	$= 6 \times \sqrt[6]{ \frac{1}{2 \cdot 2 \cdot 3 \cdot 3 \cdot 3} \frac{x}{y} \frac{y}{z} \frac{z}{x} }$

	$= 6 \times \sqrt[6]{\frac{1}{2 \cdot 2 \cdot 3 \cdot 3 \cdot 3}}$

	$= 2 \cdot 3 \cdot 2^{-1/3} \cdot 3^{-1/2}$

	$= 2^{2/3} \cdot 3^{1/2}$

Further, we know that the two sides are equal exactly when all the terms of the mean are equal:

$f(x,y,z) = 2^{2/3} \cdot 3^{1/2} \quad when \quad \frac{x}{y} = \frac{1}{2} \sqrt{\frac{y}{z}} = \frac{1}{3} \sqrt[3]{\frac{z}{x}}.$

Generalization

This inequality is a special case of Muirhead's inequality.

Categories: Inequalities

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03-10-2013 05:06:04
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