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which may be compactly written in vector and matrix notation as:
where is a row vector, xT is the transpose of x (a column vector), Q is a matrix and P is a -dimensional row vector and R a scalar constant. The values Q, P and R are often taken to be over real numbers or complex numbers, but a quadric may be defined over any field.
Quadrics in the Euclidean plane are those of dimension D = 1, which is to say that they are plane curves. In this case, one talks of conic sections, or conics.
Circle (e = 0), ellipse (e = 0.5), parabola (e = 1), and hyperbola (e = 2) with fixed focus F and directrix.
In three-dimensional Euclidean space, quadrics have dimension D = 2, and are known as quadric surfaces. They are classified and named by their orbits under affine transformations. More precisely, if an affine transformation maps a quadric onto another one, they belong to the same class, and share the same name and many properties.
where the are either 1, -1 or 0, except which takes only the value 0 or 1.
Each of these 17 normal forms
corresponds to a single orbit under affine transformations. In three cases there are no real points: (imaginary ellipsoid), (imaginary elliptic cylinder), and (pair of complex conjugate parallel planes, a reducible quadric). In one case, the imaginary cone, there is a single point (). If one has a line (in fact two complex conjugate intersecting planes). For one has two intersecting planes (reducible quadric). For one has a double plane. For one has two parallel planes (reducible quadric).
Thus, among the 17 normal forms, there are nine true quadrics: a cone, three cylinders (often called degenerate quadrics) and five non-degenerate quadrics (ellipsoid, paraboloids and hyperboloids), which are detailed in the following tables. The eight remaining quadrics are the imaginary ellipsoid (no real point), the imaginary cylinder (no real point), the imaginary cone (a single real point), and the reducible quadrics, which are decomposed in two planes; there are five such decomposed quadrics, depending whether the planes are distinct or not, parallel or not, real or complex conjugate.
When two or more of the parameters of the canonical equation are equal, one gets a quadric of revolution, which remains invariant when rotated around an axis (or infinitely many axes, in the case of the sphere).
Quadrics of revolution
Oblate and prolate spheroids (special cases of ellipsoid)
Circular cylinder (special case of elliptic cylinder)
Definition and basic properties
A quadric is the set of zeros of a polynomial of degree two. When not specified otherwise, the polynomial is supposed to have real coefficients, and the zeros are points in a Euclidean space. However, most properties remain true when the coefficients belong to any field and the points belong in an affine space. As usually in algebraic geometry, it is often useful to consider points over an algebraically closed field containing the polynomial coefficients, generally the complex numbers, when the coefficients are real.
If A is the (n + 1)×(n + 1) matrix that has the as entries, and
then the equation may be shortened in the matrix equation
The equation of the projective completion of this quadric is
These equations define a quadric as an algebraic hypersurface of dimensionn - 1 and degree two in a space of dimension n.
Normal form of projective quadrics
The quadrics can be treated in a uniform manner by introducing homogeneous coordinates on a Euclidean space, thus effectively regarding it as a projective space. Thus if the original (affine) coordinates on RD+1 are
one introduces new coordinates on RD+2
related to the original coordinates by . In the new variables, every quadric is defined by an equation of the form
where the coefficients aij are symmetric in i and j. Regarding Q(X) = 0 as an equation in projective space exhibits the quadric as a projective algebraic variety. The quadric is said to be non-degenerate if the quadratic form is non-singular; equivalently, if the matrix (aij) is invertible.
by means of a suitable projective transformation (normal forms for singular quadrics can have zeros as well as ±1 as coefficients). For surfaces in space (dimension D = 2) there are exactly three nondegenerate cases:
The first case is the empty set.
The second case generates the ellipsoid, the elliptic paraboloid or the hyperboloid of two sheets, depending on whether the chosen plane at infinity cuts the quadric in the empty set, in a point, or in a nondegenerate conic respectively. These all have positive Gaussian curvature.
The third case generates the hyperbolic paraboloid or the hyperboloid of one sheet, depending on whether the plane at infinity cuts it in two lines, or in a nondegenerate conic respectively. These are doubly ruled surfaces of negative Gaussian curvature.
The degenerate form
generates the elliptic cylinder, the parabolic cylinder, the hyperbolic cylinder, or the cone, depending on whether the plane at infinity cuts it in a point, a line, two lines, or a nondegenerate conic respectively. These are singly ruled surfaces of zero Gaussian curvature.
We see that projective transformations don't mix Gaussian curvatures of different sign. This is true for general surfaces. 
The definition of a projective quadric in a real projective space (see above) can be formally adopted defining a projective quadric in an n-dimensional projective space over a field. In order to omit dealing with coordinates a projective quadric is usually defined starting with a quadratic form on a vector space 
For the intersection of a line with a quadric the familiar statement is true:
For an arbitrary line the following cases occur:
a) and is called exterior line or
b) and is called tangent line or
b?) and is called tangent line or
c) and is called secant line.
Let be a line, which intersects at point and is a second point on .
From one gets
I) In case of the equation holds and it is
for any . Hence either
for any or for any, which proves b) and b').
II) In case of one gets and the equation
has exactly one solution .
Hence: , which proves c).
Additionally the proof shows:
A line through a point is a tangent line if and only if .
In the classical cases or there exists only one radical, because of and and are closely connected. In case of the quadric is not determined by (see above) and so one has to deal with two radicals:
a) is a projective subspace. is called f-radical of quadric .
b) is called singular radical or -radical of .
c) In case of one has .
A quadric is called non-degenerate if .
Examples in (see above): (E1): For (conic) the bilinear form is
In case of the polar spaces are never . Hence .
In case of the bilinear form is reduced to
and . Hence
In this case the f-radical is the common point of all tangents, the so called knot.
In both cases and the quadric (conic) ist non-degenerate. (E2): For (pair of lines) the bilinear form is and the intersection point.
In this example the quadric is degenerate.
For the index of a non-degenerate quadric in the following is true:
Let be a non-degenerate quadric in , and its index.
In case of quadric is called sphere (or oval conic if ).
In case of quadric is called hyperboloid (of one sheet).
a) Quadric in with form is non-degenerate with index 1.
b) If polynomial is irreducible over the quadratic form gives rise to a non-degenerate quadric in of index 1 (sphere). For example: is irreducible over (but not over !).
c) In the quadratic form generates a hyperboloid.
Generalization of quadrics: quadratic sets
It is not reasonable to extent formally the definition of quadrics to spaces over genuine skew fields (division rings). Because one would get secants bearing more than 2 points of the quadric which is totally different from usual quadrics . The reason is the following statement.
There are generalizations of quadrics: quadratic sets. A quadratic set is a set of points of a projective space with the same geometric properties as a quadric: every line intersects a quadratic set in at most two points or is contained in the set.