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Two or more lines are said to be concurrent if they intersect in a single point. Two lines concur if their trilinear coordinates satisfy |l_1 m_1 n_1; l_2 m_2 n_2; l_3 m_3 ...

A requirement necessary for a given statement or theorem to hold. Also called a criterion.

The formal term in propositional calculus for the connective implies.

A set X is called a "cone" with vertex at the origin if for any x in X and any scalar a>=0, ax in X.

Confocal conics are conic sections sharing a common focus. Any two confocal central conics are orthogonal (Ogilvy 1990, p. 77).

Confocal parabolas are parabolas sharing a common focus.

A topological structure which interpolates between contact structures and codimension-one foliations.

The five of Hilbert's axioms which concern geometric equivalence.

An equation of the form f(x)=b (mod m), (1) where the values of 0<=x<m for which the equation holds are sought. Such an equation may have none, one, or many solutions. There ...

A transformation of the form g=D^(T)etaD, where det(D)!=0 and det(D) is the determinant. Isometries are also called congruence transformations.