RootOf

Set of roots of a polynomial

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Syntax

RootOf(f, x)
RootOf(f)

Description

RootOf(f, x) represents the symbolic set of roots of the polynomial f(x) with respect to the indeterminate x.

RootOf serves as a symbolic representation of the zero set of a polynomial. Since it is generally impossible to represent the roots of a polynomial in terms of radicals, RootOf is often the only possible way to represent the roots symbolically. RootOf mainly occurs in the output of solve or related functions; see Example 3.

The parameter f must be either a polynomial, or an arithmetical expression representing a polynomial in x, or an equation p=q, where p and q are arithmetical expressions representing polynomials in x. In the latter case, RootOf represents the roots of p-q with respect to x.

The polynomial f need not be irreducible or even square-free. If f has multiple roots, RootOf represents each of the roots with its multiplicity.

If x is omitted, then f must be an arithmetical expression or polynomial equation containing exactly one indeterminate, and RootOf represents the roots with respect to this indeterminate.

x need not be an identifier or indexed identifier: it may be any expression that is neither rational nor constant.

If f contains only one indeterminate, then you can apply float to the RootOf object to obtain a set of floating-point approximations for all roots; see Example 3.

Examples

Example 1

Each of the following calls represents the roots of the polynomial x3 - x2 with respect to x, i.e., the set {0, 1}:

RootOf(x^3 - x^2, x), RootOf(x^3 = x^2, x)

RootOf(x^3 - x^2), RootOf(x^3 = x^2)

RootOf(poly(x^3 - x^2, [x]), x)

In general, however, RootOf is only used when no explicit symbolic representation of the roots is possible.

Example 2

The first argument of RootOf may contain parameters:

RootOf(y*x^2 - x + y^2, x)

The set of roots of a polynomial is treated like an expression. For example, it may be differentiated with respect to a free parameter. The result is the set of derivatives of the roots; it is expressed in terms of RootOf, by giving a minimal polynomial:

diff(%, y)

For reducible polynomials, the result may be a multiple of the correct minimal polynomial.

Example 3

solve returns RootOf objects when the roots of a polynomial cannot be expressed in terms of radicals:

solve(x^5 + x + 7, x)

You can apply the function float to obtain floating-point approximations of all roots:

float(%)

Example 4

The function sum is able to compute sums over all roots of a given polynomial:

sum(i^2, i = RootOf(x^3 + a*x^2 + b*x + c, x))

sum(1/(z + i), i = RootOf(x^4 - y*x + 1, x))

Example 5

A RootOf object represents the set of all roots. One can address the individual roots via indexed calls:

RootOf(z^3 - 1, z)[i] $ i = 1..3

float(RootOf(z^3 - 1, z)[i]) $ i = 1..3

Parameters

f

A polynomial, an arithmetical expression representing a polynomial in x, or a polynomial equation in x

x

The indeterminate: typically, an identifier or indexed identifier

Return Values

Symbolic RootOf call, i.e., an expression of type "RootOf".

See Also

MuPAD Functions