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Let A be a C^*-algebra, then an element a in A is called normal if aa^*=a^*a.

The study of numbers for the supposed purpose of predicting future events or seeking connections with the occult.

Except for convex polygons, every simple polygon has at least one mouth.

A broad area of mathematics connected with functional analysis, differential equations, index theory, representation theory, and mathematical physics.

A tree is planted at each lattice point in a circular orchard which has center at the origin and radius r. If the radius of trees exceeds 1/r units, one is unable to see out ...

A point which lies on at least one ordinary line is called an ordinary point, or sometimes a regular point.

A surface which is homeomorphic to a finite collection of spheres, each with a finite number of handles, cross-handles, cross-caps, and perforations. A preliminary version of ...

Given four points, A, B, C, and H, let H be the orthocenter of DeltaABC. Then A is the orthocenter DeltaHBC, B is the orthocenter of DeltaHAC, and C is the orthocenter of ...

The Pappus configuration is the 9_3 configuration illustrated above that appears in Pappus's hexagon theorem. It is one of the three 9_3 configurations. The Levi graph of the ...

AW, AB, and AY in the above figure are in a harmonic range.