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A group of four elements, also called a quadruplet or tetrad.

Let R be a commutative ring. A category C is called an R-category if the Hom-sets of C are R-modules.

A robust estimation based on a rank test.

The distance from the center of a circle to its perimeter, or from the center of a sphere to its surface. The radius is equal to half the diameter.

When a measure lambda is absolutely continuous with respect to a positive measure mu, then it can be written as lambda(E)=int_Efdmu. By analogy with the first fundamental ...

A knot diagram in which none of the crossings are reducible.

A regular local ring is a local ring R with maximal ideal m so that m can be generated with exactly d elements where d is the Krull dimension of the ring R. Equivalently, R ...

The sum of the reciprocals of roots of an equation equals the negative coefficient of the linear term in the Maclaurin series.

A knot which tightens around an object when strained but slackens when the strain is removed. Running knots are sometimes also known as slip knots or nooses.

A cross also called the tau cross or crux commissa, illustrated schematically above in polyomino form.