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This case always occurs as a degenerate conic in a pencil of circles. However, in other contexts it is not considered as a degenerate conic, as its equation is not of degree 2. The case of coincident lines occurs if and only if the rank of the 3×3 matrix is 1; in all other degenerate cases its rank is 2. [3]: p.108
In such a degenerate case, the solution set is said to be degenerate. For some classes of composite objects, the degenerate cases depend on the properties that are specifically studied. In particular, the class of objects may often be defined or characterized by systems of equations. In most scenarios, a given class of objects may be defined by ...
If a set of points is not in general linear position, it is called a degenerate case or degenerate configuration, which implies that they satisfy a linear relation that need not always hold. A fundamental application is that, in the plane, five points determine a conic, as long as the points are in general linear position (no three are collinear).
To distinguish the degenerate cases from the non-degenerate cases (including the empty set with the latter) using matrix notation, let β be the determinant of the 3 × 3 matrix of the conic section—that is, β = (AC − B 2 / 4 )F + BED − CD 2 − AE 2 / 4 ; and let α = B 2 − 4AC be the discriminant.
If the conic is non-degenerate, the conjugates of a point always form a line and the polarity defined by the conic is a bijection between the points and lines of the extended plane containing the conic (that is, the plane together with the points and line at infinity). If the point p lies on the conic Q, the polar line of p is the tangent line ...
It includes real circles, imaginary circles, and two degenerate point circles called the Poncelet points of the pencil. Each point in the plane belongs to exactly one circle of the pencil. A parabolic pencil (as a limiting case) is defined where two generating circles are tangent to each other at a single point. It consists of a family of real ...
2 The number of lines meeting 4 general lines in space; 8 The number of circles tangent to 3 general circles (the problem of Apollonius). 27 The number of lines on a smooth cubic surface (Salmon and Cayley) 2875 The number of lines on a general quintic threefold; 3264 The number of conics tangent to 5 plane conics in general position
A limiting case is sometimes a degenerate case in which some qualitative properties differ from the corresponding properties of the generic case. For example: A point is a degenerate circle, whose radius is zero. A parabola can degenerate into two distinct or coinciding parallel lines. An ellipse can degenerate into a single point or a line ...