# Difference between revisions of "Polynomial"

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− | A polynomial is a [[function]] in one or more [[variable]]s that consists of a sum of variables raised to [[integer|integral]] powers and multiplied by | + | A '''polynomial''' is a [[function]] in one or more [[variable]]s that consists of a sum of variables raised to [[integer|integral]] powers and multiplied by [[coefficient]]s. |

For example, these are polynomials: | For example, these are polynomials: | ||

− | * <math>4x^2 + 6x - 9</math>, in the variable x | + | * <math>4x^2 + 6x - 9</math>, in the variable <math>x</math> |

− | * <math>x^3 + 3x^2y + 3xy^2 + y^3</math>, in the variables x and y | + | * <math>x^3 + 3x^2y + 3xy^2 + y^3</math>, in the variables <math>x</math> and <math>y</math> |

− | * <math>5x^4 - 2x^2 + 9</math>, in the variable x | + | * <math>5x^4 - 2x^2 + 9</math>, in the variable <math>x</math> |

* <math>\sin^2{x} + 5</math>, in the variable <math>\sin x</math> | * <math>\sin^2{x} + 5</math>, in the variable <math>\sin x</math> | ||

Line 10: | Line 10: | ||

* <math>\sin^2{x} + 5</math> | * <math>\sin^2{x} + 5</math> | ||

* <math>\frac{4x+3}{2x-9}</math> | * <math>\frac{4x+3}{2x-9}</math> | ||

− | are functions, but ''not'' polynomials, in the variable x | + | are functions, but ''not'' polynomials, in the variable <math>x</math> |

==Introductory Topics== | ==Introductory Topics== | ||

+ | |||

+ | The simplest piece of information that one can have about a polynomial of one variable is the highest power of the variable which appears in the polynomial. This number is known as the ''degree'' of the polynomial and is written <math>\deg(P)</math>. For instance, <math>\deg(x^2 + 3x + 4) = 2</math> and <math>\deg(x^5 - 1) = 5</math>. The degree, together with the coefficient of the largest term, provides a surprisingly large amount of information about the polynomial: how it behaves in the [[limit]] as the variable approaches positive or negative infinity and how many roots it has. | ||

===A More Precise Definition=== | ===A More Precise Definition=== | ||

− | A polynomial in one variable is a function <math>P(x) = a_nx^n + a_{n-1}x^{n-1} + \cdots + a_2x^2 + a_1x + a_0</math>. Here, <math>a_i</math> is the <math>i</math>th coefficient and <math>a_n \neq 0</math>. The integer <math>n</math> is | + | A polynomial in one variable is a function <math>P(x) = a_nx^n + a_{n-1}x^{n-1} + \cdots + a_2x^2 + a_1x + a_0</math>. Here, <math>a_i</math> is the <math>i</math>th coefficient and <math>a_n \neq 0</math>. The integer <math>n</math> is the degree of the polynomial. Often, the leading coefficient of a polynomial will be equal to 1. In this case, we say we have a ''monic'' polynomial. |

===Finding Roots of Polynomials=== | ===Finding Roots of Polynomials=== | ||

Line 22: | Line 24: | ||

====What is a root?==== | ====What is a root?==== | ||

− | A [[root]] is a value for a variable that will make the polynomial equal zero. For an example, 2 is a root of <math>x^2 - 4</math> because <math>2^2 - 4 = 0</math>. For some polynomials, you can easily set the polynomial equal to zero and solve | + | A [[root]] is a value for a variable that will make the polynomial equal zero. For an example, 2 is a root of <math>x^2 - 4</math> because <math>2^2 - 4 = 0</math>. For some polynomials, you can easily set the polynomial equal to zero and solve or otherwise find roots, but in some cases it is much more complicated. |

====The Fundamental Theorem of Algebra==== | ====The Fundamental Theorem of Algebra==== | ||

− | The [[ | + | The [[Fundamental Theorem of Algebra]] states that any polynomial with [[complex number|complex]] coefficients can be written as |

− | <math>P(x) = k(x-x_1)(x-x_2)\cdots(x-x_n)</math> where <math>k</math> is a constant, the <math>x_i</math> are (not necessarily distinct) complex numbers and <math>n</math> is the | + | <math>P(x) = k(x-x_1)(x-x_2)\cdots(x-x_n)</math> where <math>k</math> is a constant, the <math>x_i</math> are (not necessarily distinct) complex numbers and <math>n</math> is the degree of the polynomial in exactly one way (not counting re-arrangements of the terms of the product). It's very easy to find the roots of a polynomial in this form because the roots will be <math>x_1,x_2,...,x_n</math>. This also tells us that the degree of a given polynomial is at least as large as the number of distinct roots of that polynomial. |

====Factoring==== | ====Factoring==== |

## Revision as of 15:22, 29 July 2006

A **polynomial** is a function in one or more variables that consists of a sum of variables raised to integral powers and multiplied by coefficients.

For example, these are polynomials:

- , in the variable
- , in the variables and
- , in the variable
- , in the variable

However,

are functions, but *not* polynomials, in the variable

## Introductory Topics

The simplest piece of information that one can have about a polynomial of one variable is the highest power of the variable which appears in the polynomial. This number is known as the *degree* of the polynomial and is written . For instance, and . The degree, together with the coefficient of the largest term, provides a surprisingly large amount of information about the polynomial: how it behaves in the limit as the variable approaches positive or negative infinity and how many roots it has.

### A More Precise Definition

A polynomial in one variable is a function . Here, is the th coefficient and . The integer is the degree of the polynomial. Often, the leading coefficient of a polynomial will be equal to 1. In this case, we say we have a *monic* polynomial.

### Finding Roots of Polynomials

#### What is a root?

A root is a value for a variable that will make the polynomial equal zero. For an example, 2 is a root of because . For some polynomials, you can easily set the polynomial equal to zero and solve or otherwise find roots, but in some cases it is much more complicated.

#### The Fundamental Theorem of Algebra

The Fundamental Theorem of Algebra states that any polynomial with complex coefficients can be written as

where is a constant, the are (not necessarily distinct) complex numbers and is the degree of the polynomial in exactly one way (not counting re-arrangements of the terms of the product). It's very easy to find the roots of a polynomial in this form because the roots will be . This also tells us that the degree of a given polynomial is at least as large as the number of distinct roots of that polynomial.

#### Factoring

Different methods of factoring can help find roots of polynomials. Consider this polynomial:

This polynomial easily factors to:

Now, the roots of the polynomial are clearly -3, -2, and 2.

#### The Rational Root Theorem

We are often interested in finding the roots of polynomials with integral coefficients. Consider such a polynomial . It can be shown that if has a rational root and this fraction is fully reduced, then is a factor of and is a factor of . This is convenient because it means we must check only a small number of cases to find all rational roots of many polynomials. It is also especially convenient when dealing with monic polynomials.

#### Descartes' Law of Signs

By the Fundamental Theorem of Algebra, the maximum number of distinct factors (not all necessarily real) of a polynomial of degree n is n. This tells us nothing about whether or not these roots are positive or negative. Decartes' Rule of Signs says that for a polynomial P(x), the number of positive roots to the equation is equal to the number of sign changes in the coefficients of the polynomial, or is less than that number by a multiple of 2. The number of negative roots to the equation is the number of sign changes in the coefficients of P(-x), or is less than that by a multiple of 2.

#### Binomial Theorem

Binomial theorem can be very useful for factoring and expanding polynomials.

## Intermediate Topics

## Olympiad Topics

## Other Resources

An extensive coverage of this topic is given in A Few Elementary Properties of Polynomials by Adeel Khan.