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A concordance between knots K_0 and K_1 in S^3 is a locally flat cylinder C=S^1×[0,1] embedded in S^3×[0,1] in such a way that the ends S^1×{1} are embedded in S^3×{i} as ...

Two or more lines which intersect in a point are said to concur.

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.