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A unit in a ring is an element u such that there exists u^(-1) where u·u^(-1)=1.

A ringoid is a set R with two binary operators, conventionally denoted addition (+) and multiplication (×), where × distributes over + left and right: a(b+c)=ab+ac and ...

Rubik's graph is the Cayley graph of Rubik's group. The graph diameter of this graph is sometimes known as God's number, and was shown in Aug. 2010 to be equal to 20 (Rokicki ...

Rubik's group is the group corresponding to possible rotations of a Rubik's Cube. There are six possible rotations, each corresponding to a generator of the group, and the ...

The Rudvalis group is the sporadic group Ru of order |Ru| = 145926144000 (1) = 2^(14)·3^3·5^3·7·13·29. (2) It is implemented in the Wolfram Language as RudvalisGroupRu[].

The divisibility test that an integer is divisible by 9 iff the sum of its digits is divisible by 9.

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.

The Rutherford constant is a mathematical constant that arises in plasma physics and is defined by K_R = sqrt(2)int_(-1)^infty([R(x)]^2)/(S[x])dx (1) = 0.8227..., (2) where ...

The Sally sequence gives the sequence of lengths of the repetitions which are avoided in the Linus sequence. The first few terms are 0, 1, 1, 2, 1, 3, 1, 1, 3, 2, 1, 6, 3, 2, ...

In order for a band-limited (i.e., one with a zero power spectrum for frequencies nu>B) baseband (nu>0) signal to be reconstructed fully, it must be sampled at a rate nu>=2B. ...