Descartes’ rule of signs

Descartes’s rule of signs is a method for determining the number of positive or negative roots of a polynomialMathworldPlanetmath.

Let p(x)=i=0maixi be a polynomial with real coefficients such that am0.

Define v to be the number of variations in sign of the sequence of coefficients am,,a0. By ’variations in sign’ we mean the number of values of n such that the sign of an differs from the sign of an-1, as n ranges from m down to 1.

For example, consider p(x)=x2-4x+4. The coefficients are 1,-4,4, so there are 2 variations in sign (since the sign of 1 differs from that of -4, which in turn differs from that of 4.)

Then the number of positive real roots of p(x) is v-2N for some integer N satisfying 0Nv2.

The number N represents the number of irreduciblePlanetmathPlanetmath factors of degree 2 in the factorization of p(x). Thus N=0 if is known that p(x) splits over the real numbers.

The number of negative roots of p(x) may be obtained by the same method by applying the rule of signs to p(-x).


This result is believed to have been first described by Réné Descartes in his 1637 work La Géométrie. In 1828, Carl Friedrich Gauss improved the rule by proving that when there are fewer roots of polynomials than there are variations of sign, the parity of the difference between the two is even.


Let p(x)=x3+3x2-x-3. Looking at the list of coefficients, we have 1,3,-1,-3, so there is only one variation in sign (from 3 to -1).

Thus v=1. Since 0N12 then we must have N=0. Thus v-2N=1 and so there is exactly one positive real root of p(x).

To find the negative roots, we examine p(-x)=-x3+3x2+x-3. The coefficient list is -1,3,1,-3, so here are are two variations in sign (from -1 to 3 and 1 to -3. Thus v=2 and so 0N22=1.

Thus we have two possible solutions, N=0 and N=1, and two possible values of v-2N. Therefore there are either two negative real roots or none at all.

We note that p(-1)=(-1)3+3(-1)2-(-1)-3=0, hence there is at least one negative root. Therefore there must be exactly two.

Title Descartes’ rule of signs
Canonical name DescartesRuleOfSigns
Date of creation 2013-03-22 14:28:31
Last modified on 2013-03-22 14:28:31
Owner PrimeFan (13766)
Last modified by PrimeFan (13766)
Numerical id 14
Author PrimeFan (13766)
Entry type Theorem
Classification msc 26C10
Classification msc 26A06