MATLAB Function Reference |

Polynomial with specified roots

**Syntax**

**Description**

```
p = poly(A)
```

where `A`

is an `n`

-by-`n`

matrix returns an `n+1`

element row vector whose elements are the coefficients of the characteristic polynomial, *.* The coefficients are ordered in descending powers: if a vector `c`

has `n+1`

components, the polynomial it represents is

```
p = poly(r)
```

where `r`

is a vector returns a row vector whose elements are the coefficients of the polynomial whose roots are the elements of `r`

.

**Remarks**

Note the relationship of this command to

which returns a column vector whose elements are the roots of the polynomial specified by the coefficients row vector `p`

. For vectors, `roots`

and `poly`

are inverse functions of each other, up to ordering, scaling, and roundoff error.

**Examples**

MATLAB displays polynomials as row vectors containing the coefficients ordered by descending powers. The characteristic equation of the matrix

is returned in a row vector by `poly`

:

The roots of this polynomial (eigenvalues of matrix `A)`

are returned in a column vector by `roots`

:

**Algorithm**

The algorithms employed for `poly`

and `roots`

illustrate an interesting aspect of the modern approach to eigenvalue computation. `poly(A)`

generates the characteristic polynomial of `A`

, and `roots(poly(A))`

finds the roots of that polynomial, which are the eigenvalues of `A`

. But both `poly`

and `roots`

use `eig`

, which is based on similarity transformations. The classical approach, which characterizes eigenvalues as roots of the characteristic polynomial, is actually reversed.

If `A`

is an `n`

-by-`n`

matrix, `poly(A)`

produces the coefficients `c(1)`

through `c(n+1)`

, with `c(1)`

`=`

`1`

, in

This recursion is easily derived by expanding the product.

It is possible to prove that `poly(A)`

produces the coefficients in the characteristic polynomial of a matrix within roundoff error of `A`

. This is true even if the eigenvalues of `A`

are badly conditioned. The traditional algorithms for obtaining the characteristic polynomial, which do not use the eigenvalues, do not have such satisfactory numerical properties.

**See Also**

polar | polyarea |

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