For a cubic polynomial with real coefficients, the discriminant is zero when two roots coincide, positive if the roots are three distinct real numbers, and negative if there is one real root and two distinct complex conjugate roots. For a real polynomial of degree 4 or greater, the discriminant is zero if and only if it has a multiple root, and positive if and only if the number of non-real roots is a multiple of 4.
The discriminant is widely used in number theory, either directly or through its generalization as the discriminant of a number field. For factoring a polynomial with integer coefficients, the standard method consists in first factoring its reduction modulo a prime number not dividing the discriminant (nor dividing the leading coefficient). In algebraic geometry, the discriminant with respect to one of the variables characterizes the points of a hypersurface where the implicit function theorem does not apply.
The term "discriminant" was coined in 1851 by the British mathematician James Joseph Sylvester.
be a polynomial of degree n (this means ), such that the coefficients belong to a field, or, more generally, to a commutative ring. The resultant of A and its derivative is a polynomial in with integer coefficients, which is the determinant of the Sylvester matrix of A and A′. The nonzero entries of the first column of the Sylvester matrix are and and the resultant is thus a multiple of So, the discriminant, up to its sign, is defined as the quotient of the resultant of A and A' by
Historically, this sign has been chosen such that, over the reals, the discriminant will be positive when all the roots of the polynomial are real. The division by may be not well defined if the ring of the coefficients contains zero divisors. Such a problem may be avoided by replacing by 1 in the first column of the Sylvester matrix before computing the determinant. In any case, the discriminant is a polynomial in with integer coefficients.
Expression in terms of the roots
When the polynomial is defined over a field, the fundamental theorem of algebra implies that it has n roots, r1, r2, ..., rn, not necessarily all distinct, in an algebraically closed extension of the field (for a polynomial with real coefficients, this algebraically closed extension is generally chosen as the field of complex numbers).
In terms of the roots, the discriminant is equal to
It is thus the square of the Vandermonde polynomial times an2n − 2.
This expression of the discriminant is often taken as a definition. It makes clear that if the polynomial has a multiple root, then its discriminant is zero, and that if all the roots are real, then the discriminant is positive.
The discriminant of a linear polynomial (degree 1) is rarely considered. If needed, it is commonly defined to be equal to 1 (using the usual conventions for the empty product and the determinant of the empty matrix). There is no common convention for the discriminant of a constant polynomial (degree 0).
For small degrees, the discriminant is rather simple (see below), but for higher degrees it becomes unwieldy. The discriminant of a general quartic has 16 terms, that of a quintic has 59 terms, and that of a sextic has 246 terms. This is OEIS sequence A007878.
The quadratic polynomial has discriminant
The square root of the discriminant appears in the quadratic formula for the roots of the quadratic polynomial:
The discriminant is zero if and only if the two roots are equal. If a, b, c are real numbers, the polynomial has two distinct real roots if the discriminant is positive, and two complex conjugate roots if it is negative.
If a, b, c are rational numbers, then the discriminant is the square of a rational number if and only if the two roots are rational numbers.
The cubic polynomial has discriminant
In particular, the polynomial has discriminant
The discriminant is zero if and only if at least two roots are equal. If the coefficients are real numbers, and the discriminant is not zero, the discriminant is positive if the roots are three distinct real numbers, and negative if there is one real root and two complex conjugate roots.
If the polynomial is irreducible and its coefficients are rational numbers (or belong to a number field), then the discriminant is a square of a rational number (or a number from the number field) if and only if the Galois group of the cubic equation is the cyclic group of order three.
The quartic polynomial has discriminant
The discriminant is zero if and only if two or more roots are equal. If the coefficients are real numbers and the discriminant is negative there are two real roots and two complex conjugate roots, likewise if the discriminant is positive the roots are either all real or all non-real.
The discriminant of a polynomial over an integral domain is zero if and only if the polynomial and its derivative have a non-constant common divisor.
In nonzero characteristic p, the discriminant is zero if and only if the polynomial is not square-free or it has an irreducible factor which not separable (that is, the irreducible factor is a polynomial in ).
Invariance under change of the variable
The discriminant of a polynomial is, up to a scaling, invariant under any projective transformation of the variable. As a projective transformation may be decomposed into a product of translations, homotheties and inversions, this results of the following formulas for simpler transformations, where P(x) denotes a polynomial in the variable x of degree n, with as leading coefficient.
- Invariance by translation:
- This results from the expression of the discriminant in terms of the roots
- Invariance by homothety:
This results of the expression in terms of the roots, or of the quasi-homogeneity of the discriminant.
- Invariance by inversion:
- Here denotes the reciprocal polynomial of P. That is, if then
Invariance under ring homomorphisms
in R[x], the homomorphism acts on A for producing the polynomial
The discriminant is invariant under in the following sense. If then
As the discriminant is defined in terms of a determinant, this property results immediately from the similar property of determinants.
If then may be zero or not. One has, when
When, as it is generally the case in algebraic geometry, one is interested only to know if a discriminant is zero or not, these properties may be summarised into:
- if and only either or
Product of polynomials
If P = QR is a product of polynomials in x, then
where Res() denotes the resultant, and p and q are the respective degrees of P and Q.
This property follows immediately by substituting the expression for the resultant, and the discriminant, in terms of the roots of the respective polynomials.
The discriminant of a polynomial of degree n is homogeneous of degree 2n − 2 in the coefficients. This can be seen two ways. In terms of the roots-and-leading-term formula, multiplying all the coefficients by λ does not change the roots, but multiplies the leading term by λ. In terms of its expression as a determinant of a (2n − 1) × (2n − 1) matrix (the Sylvester matrix) divided by an, the determinant is homogeneous of degree 2n − 1 in the entries, and dividing by an makes the degree 2n − 2.
The discriminant of a polynomial of degree n is homogeneous of degree n(n − 1) in the roots. This follows from the expression of the discriminant in terms of the roots, which is the product of a constant and squared differences of roots.
The discriminant of a polynomial of degree n is quasi-homogeneous of degree n(n − 1) in the coefficient, if, for every i, the coefficient of is given the weight n − i. It is also quasi-homogeneous of the same degree, if, for every i, the coefficient of is given the weight This is a consequence of the general fact that every polynomial which is homogeneous and symmetric in the roots may be expressed as a quasi-homogeneous polynomial in the elementary symmetric functions of the roots.
Consider the polynomial
It follows from what precedes that the exponents in every monomial a0i0. ..., anin appearing in the discriminant satisfies the two equations
and also the equation
which is obtained by subtracting the second equation to the first one multiplied by n.
This restricts the possible terms in the discriminant. For the general quadratic polynomial there are only two possibilities and two terms in the discriminant, while the general homogeneous polynomial of degree two in three variables has 6 terms. For the general cubic polynomial, there are five possibilities and five terms in the discriminant, while the general homogeneous polynomial of degree 4 in 5 variables has 70 terms
For higher degrees, there may monomials which satisfy above equations and do not appear in the discriminant. The first example is for the quartic polynomial ax4 + bx3 + cx2 + dx + e, in which case the monomial bc4d satisfies the equations without appearing in the discriminant.
In this section, all polynomials have real coefficients.
It has been seen in § Low degrees that the sign of the discriminant provides a full information on the nature of the roots for polynomials of degree 2 and 3. For higher degrees, the information provided by the discriminant is less complete, but still useful. More precisely, for a polynomial of degree n, one has:
- The polynomial has a multiple root if and only if its discriminant is zero.
- If the discriminant is positive, the number of non-real roots is a multiple of 4. That is, there is a nonnegative integer k ≤ n/4 such there are 2k pairs of complex conjugate roots and n − 4k real roots.
- If the discriminant is negative, the number of non-real roots is not a multiple of 4. That is, there is a nonnegative integer k ≤ (n − 2)/4 such there are 2k + 1 pairs of complex conjugate roots and n − 4k + 2 real roots.
Homogeneous bivariate polynomial
be a homogeneous polynomial of degree n in two indeterminates.
Supposing, for the moment, that and are both nonzero, one has
Denoting this quantity by one has
Because of these properties, the quantity is called the discriminant or the homogeneous discriminant of A.
If and may be zero, the polynomials A(x, 1) and A(1, y) may have a degree smaller than n. In this case, above formulas and definition remain valid, if the discriminants are computed as if all polynomials would have the degree n. This means that the discriminants must be computed with and indeterminate, the substitution for them of their actual values being done after this computation. Equivalently, the formulas of § Invariance under ring homomorphisms must be used.
Use in algebraic geometry
The typical use of discriminants in algebraic geometry is for studying algebraic curve and, more generally algebraic hypersurfaces. Let V be such a curve or hypersurface; V is defined as the zero set of a multivariate polynomial. This polynomial may be considered as a univariate polynomial in one of the indeterminates, with polynomials in the other indeterminates as coefficients. The discriminant with respect to the selected indeterminate defines a hypersurface W in the space of the other indeterminates. The points of W are exactly the projection of the points of V (including the points at infinity), which either are singular or have a tangent hyperplane that is parallel to the axis of the selected indeterminate.
For example, let f be a bivariate polynomial in X and Y with real coefficients, such that f = 0 is the implicit equation of a plane algebraic curve. Viewing f as a univariate polynomial in Y with coefficients depending on X, then the discriminant is a polynomial in X whose roots are the X-coordinates of the singular points, of the points with a tangent parallel to the Y-axis and of some of the asymptotes parallel to the Y-axis. In other words, the computation of the roots of the Y-discriminant and the X-discriminant allows one to compute all of the remarkable points of the curve, except the inflection points.
There are two classes of the concept of discriminant. The first class is the discriminant of an algebraic number field, which, in some cases including quadratic fields, is the discriminant of a polynomial defining the field.
Discriminants of the second class arise for problems depending on coefficients, when degenerate instances or singularities of the problem are characterized by the vanishing of a single polynomial in the coefficients. This is the case for the discriminant of a polynomial, which is zero when two roots collapse. Most of the cases, where such a generalized discriminant is defined, are instances of the following.
Let A be a homogeneous polynomial in n indeterminates over a field of characteristic 0, or of a prime characteristic that does not divides the degree of the polynomial. The polynomial A defines a projective hypersurface, which has singular points if and only the n partial derivatives of A have a nontrivial common zero. This is the case if and only if the multivariate resultant of these partial derivatives is zero, and this resultant may be considered as the discriminant of A. However, because of the integer coefficients resulting of the derivation, this multivariate resultant may be divisible by a power of n, and it is better to take, as a discriminant, the primitive part of the resultant, computed with generic coefficients. The restriction on the characteristic is needed, as, otherwise, a common zero of the partial derivative is not necessarily a zero of the polynomial (see Euler's identity for homogeneous polynomials).
In the case of a homogeneous bivariate polynomial of degree d, this general discriminant is times the discriminant defined in § Homogeneous bivariate polynomial. Several other classical types of discriminants, that are instances of the general definition are described in next sections.
or, in matrix form,
The Hessian determinant of Q is times its discriminant. The multivariate resultant of the partial derivatives of Q is equal to its Hessian determinant. So, the discriminant of a quadratic form is a special case of the above general definition of a discriminant.
The discriminant of a quadratic form is invariant under linear changes of variables (that is a change of basis of the vector space on which the quadratic form is defined) in the following sense: a linear change of variables is defined by a nonsingular matrix S, changes the matrix A into and thus multiplies the discriminant by the square of the determinant of S. Thus the discriminant is well defined only up to the multiplication by a square. In other words, the discriminant of a quadratic form over a field K is an element of K/(K×)2, the quotient of the multiplicative monoid of K by the subgroup of the nonzero squares (that is, two elements of K are in the same equivalence class if one is the product of the other by a nonzero square). It follows that over the complex numbers, a discriminant is equivalent to 0 or 1. Over the real numbers, a discriminant is equivalent to −1, 0, or 1. Over the rational numbers, a discriminant is equivalent to a unique square-free integer.
By a theorem of Jacobi, a quadratic form over a field of characteristic different from 2 can be expressed, after a linear change of variables, in diagonal form as
More precisely, a quadratic forms on may be expressed as a sum
where the Li are independent linear forms and n is the number of the variables (some of the ai may be zero). Equivalently, for any symmetric matrix A, there is an elementary matrix S such that is a diagonal matrix. Then the discriminant is the product of the ai, which is well-defined as a class in K/(K×)2.
Geometrically, the discriminant of a quadratic form in three variables is the equation of a quadratic projective curve. The discriminant is zero if and only if the curve is decomposed in lines (possibly over an algebraically closed extension of the field).
A quadratic form in four variable is the equation of a projective surface. The surface has a singular point if and only its discriminant is zero. In this case, either the surface may be decomposed in planes, or it has a unique singular point, and is a cone or a cylinder. Over the reals, if the discriminant is positive, then the surface either has no real point or has everywhere a negative Gaussian curvature. If the discriminant is negative, the surface has real points, and has a negative Gaussian curvature.
where a, b, c, d, e, f are real numbers.
Two quadratic forms, and thus two discriminants may be associated to a conic section.
The first quadratic form is
Its discriminant is the determinant
It is zero if the conic section degenerates into two lines, a double line or a single point.
and determines the shape of the conic section. If this discriminant is negative, the curve either has no real points, or is an ellipse or a circle, or, if degenerated, is reduced to a single point. If the discriminant is zero, the curve is a parabola, or, if degenerated, a double line or two parallel lines. If the discriminant is positive, the curve is a hyperbola, or, if degenerated, a pair of intersecting lines.
Real quadric surfaces
A real quadric surface in the Euclidean space of dimension three is a surface that may be defined as the zeros of a polynomial of degree two in three variables. As for the conic sections there are two discriminants that may be naturally defined. Both are useful for getting information on the nature of a quadric surface.
Let be a polynomial of degree two in three variables that defines a real quadric surface. The first associated quadratic form, depends on four variables, and is obtained by homogenizing P; that is
Let us denote its discriminant by
The second quadratic form, depends on three variables, and consists of the terms of degree two of P; that is
Let us denote its discriminant by
If the surface has a singular point, possibly at infinity. If there is only one singular point, the surface is a cylinder or a cone. If there are several singular points the surface consists of two planes, a double plane or a single line.
When the sign of if not 0, does not provide any useful information, as changing P into −P does not change the surface, but changes the sign of However, if and the surface is a paraboloid, which is elliptic of hyperbolic, depending on the sign of
Discriminant of an algebraic number field
- Sylvester, J. J. (1851). "On a remarkable discovery in the theory of canonical forms and of hyperdeterminants". Philosophical Magazine. 4th series. 2: 391–410.
Sylvester coins the word "discriminant" on page 406.
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