Descartes' rule of signs

Short description: Link between the number of positive roots of a polynomial and the signs of its coefficients

In mathematics, Descartes' rule of signs, first described by René Descartes in his work La Géométrie, is a technique for getting information on the number of positive real roots of a polynomial. It asserts that the number of positive roots is at most the number of sign changes in the sequence of polynomial's coefficients (omitting the zero coefficients), and that the difference between these two numbers is always even. This implies, in particular, that if the number of sign changes is zero or one, then there are exactly zero or one positive roots, respectively.

By a homographic transformation of the variable, one may use Descartes' rule of signs for getting a similar information on the number of roots in any interval. This is the basic idea of Budan's theorem and Budan–Fourier theorem. By repeating the division of an interval into two intervals, one gets eventually a list of disjoints intervals containing together all real roots of the polynomial, and containing each exactly one real root. Descartes rule of signs and homographic transformations of the variable are, nowadays, the basis of the fastest algorithms for computer computation of real roots of polynomials (see Real-root isolation).

Descartes himself used the transformation x → –x for using his rule for getting information of the number of negative roots.

Descartes' rule of signs

Positive roots

The rule states that if the nonzero terms of a single-variable polynomial with real coefficients are ordered by descending variable exponent, then the number of positive roots of the polynomial is either equal to the number of sign changes between consecutive (nonzero) coefficients, or is less than it by an even number. A root of multiplicity k is counted as k roots.

In particular, if the number of sign changes is zero or one, the number of positive roots equals the number of sign changes

Negative roots

As a corollary of the rule, the number of negative roots is the number of sign changes after multiplying the coefficients of odd-power terms by −1, or fewer than it by an even number. This procedure is equivalent to substituting the negation of the variable for the variable itself. For example, the negative roots of [math]\displaystyle{ ax^3+bx^2+cx+d }[/math] are the positive roots of

[math]\displaystyle{ a(-x)^3+b(-x)^2+c(-x)+d = -ax^3+bx^2-cx+d. }[/math]

Thus, applying Descartes' rule of signs to this polynomial gives the number of negative roots of the original polynomial.

Example: real roots

The polynomial

[math]\displaystyle{ f(x) = + x^3 + x^2 - x - 1 }[/math]

has one sign change between the second and third terms (the sequence of signs is (+, +, –, –). Therefore it has exactly one positive root. To find the number of negative roots, change the signs of the coefficients of the terms with odd exponents, i.e., apply Descartes' rule of signs to the polynomial [math]\displaystyle{ f(-x) }[/math], to obtain the polynomial

[math]\displaystyle{ f(-x)= - x^3 + x^2 + x - 1 }[/math]

This polynomial has two sign changes (the sequence signs is (–, +, +, –)), meaning that this second polynomial has two or zero positive roots; thus the original polynomial has two or zero negative roots.

In fact, the factorization of the first polynomial is

[math]\displaystyle{ f(x)=(x + 1)^{2}(x - 1), }[/math]

so the roots are –1 (twice) and +1 (once).

The factorization of the second polynomial is

[math]\displaystyle{ f(-x)=-(x - 1)^{2}(x + 1), }[/math]

So here, the roots are +1 (twice) and –1 (once), the negation of the roots of the original polynomial.

Nonreal roots

Any nth degree polynomial has exactly n roots in the complex plane, if counted according to multiplicity. So if f(x) is a polynomial which does not have a root at 0 (that is a polynomial with a nonzero constant term) then the minimum number of nonreal roots is equal to

[math]\displaystyle{ n-(p+q), }[/math]

where p denotes the maximum number of positive roots, q denotes the maximum number of negative roots (both of which can be found using Descartes' rule of signs), and n denotes the degree of the equation.

Example: some zero coefficients and nonreal roots

The polynomial

[math]\displaystyle{ f(x) = x^3-1 , }[/math]

has one sign change; so the number of positive real roots is one. As

[math]\displaystyle{ f(-x) = -x^3-1 , }[/math]

has no sign change, the original polynomial has no negative real roots. So the number of nonreal roots is

[math]\displaystyle{ 3 - (1+0) = 2 \, . }[/math]

Since nonreal roots of a polynomial with real coefficients must occur in conjugate pairs, it results can that x³ − 1 has exactly two nonreal roots and one real root, which is positive.

Special case

The subtraction of only multiples of 2 from the maximal number of positive roots occurs because the polynomial may have nonreal roots, which always come in pairs since the rule applies to polynomials whose coefficients are real. Thus if the polynomial is known to have all real roots, this rule allows one to find the exact number of positive and negative roots. Since it is easy to determine the multiplicity of zero as a root, the sign of all roots can be determined in this case.

Generalizations

If the real polynomial P has k real positive roots counted with multiplicity, then for every a > 0 there are at least k changes of sign in the sequence of coefficients of the Taylor series of the function e^axP(x). For a sufficiently large, there are exactly k such changes of sign.

In the 1970s Askold Khovanskii developed the theory of fewnomials that generalises Descartes' rule. The rule of signs can be thought of as stating that the number of real roots of a polynomial is dependent on the polynomial's complexity, and that this complexity is proportional to the number of monomials it has, not its degree. Khovanskiǐ showed that this holds true not just for polynomials but for algebraic combinations of many transcendental functions, the so-called Pfaffian functions.

See also

• Sturm's theorem – Count of the roots of a polynomial in an interval, without computing them
• Rational root theorem – Relationship between the rational roots of a polynomial and its extreme coefficients
• Geometrical properties of polynomial roots – Geometry of the location of polynomial roots
• Gauss–Lucas theorem – Geometric relation between the roots of a polynomial and those of its derivative

External links

• Descartes' Rule of Signs – Proof of the rule
• Descartes' Rule of Signs – Basic explanation