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A condition which must hold for a result to be true, but which does not guarantee it to be true. If a condition is both necessary and sufficient, then the result is said to ...

A negative definite matrix is a Hermitian matrix all of whose eigenvalues are negative. A matrix m may be tested to determine if it is negative definite in the Wolfram ...

A negative matrix is a real or integer matrix (a)_(ij) for which each matrix element is a negative number, i.e., a_(ij)<0 for all i, j. Negative matrices are therefore a ...

Let V be a complete normal variety, and write G(V) for the group of divisors, G_n(V) for the group of divisors numerically equal to 0, and G_a(V) the group of divisors ...

The second-order ordinary differential equation satisfied by the Neumann polynomials O_n(x).

An algebra, also called a nilalgebra, consisting only of nilpotent Elements.

Any cubic curve that passes through eight of the nine intersections of two given cubic curves automatically passes through the ninth.

Any irreducible curve may be carried by a factorable Cremona transformation into one with none but ordinary singular points.

A geometry in which Archimedes' axiom does not hold.

The nonagon, also known as an enneagon, is a 9-sided polygon. Although the term "enneagon" is perhaps preferable (since it uses the Greek prefix and suffix instead of the ...