Miscellaneous Functions - GNU Octave
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### 28.6 Miscellaneous Functions

— Function File: poly (A)
— Function File: poly (x)

> If A is a square N-by-N matrix, `poly (`A`)` is the row vector of the coefficients of `det (z * eye (N) - A)`, the characteristic polynomial of A. For example, the following code finds the eigenvalues of A which are the roots of `poly (`A`)`.

>               roots(poly(eye(3)))
>               ⇒ 1.00001 + 0.00001i
>               ⇒ 1.00001 - 0.00001i
>               ⇒ 0.99999 + 0.00000i

> In fact, all three eigenvalues are exactly 1 which emphasizes that for numerical performance the `eig` function should be used to compute eigenvalues.

> If x is a vector, `poly (`x`)` is a vector of the coefficients of the polynomial whose roots are the elements of x. That is, if c is a polynomial, then the elements of d`= roots (poly (`c`))` are contained in c. The vectors c and d are not identical, however, due to sorting and numerical errors.

> See also: roots, eig.

— Function File: polyout (c)
— Function File: polyout (c, x)
— Function File: str = polyout (...)

> Write formatted polynomial

>                  c(x) = c(1) * x^n + ... + c(n) x + c(n+1)

> and return it as a string or write it to the screen (if nargout is zero). x defaults to the string `"s"`.

> See also: polyreduce.

— Function File: polyreduce (c)

> Reduce a polynomial coefficient vector to a minimum number of terms by stripping off any leading zeros.

> See also: polyout.