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Train shunting puzzles, also often called railway shunting puzzles or railroad switching puzzles, are a type of puzzle. Shunting puzzles usually consist of a specific track layout, a set of initial conditions (typically the starting place of each item of rolling stock ), a defined goal (the finishing place of each rolling stock item), and rules ...
A double switch, or double slip—the points are set to connect the upper left and lower right tracks. A double slip switch (double slip) is a narrow-angled diagonal flat crossing of two lines combined with four pairs of points in such a way as to allow vehicles to change from one straight track to the other, alternatively to going straight ...
The numbers within the triangle count partitions in which a given element is the largest singleton. The number of partitions of an n-element set into exactly k (non-empty) parts is the Stirling number of the second kind S(n, k). The number of noncrossing partitions of an n-element set is the Catalan number
Any set of m integers, no two of which are congruent modulo m, is called a complete residue system modulo m. The least residue system is a complete residue system, and a complete residue system is simply a set containing precisely one representative of each residue class modulo m. [4] For example, the least residue system modulo 4 is {0, 1, 2, 3}.
In group and set theory, many algebraic structures are called commutative when certain operands satisfy the commutative property. In higher branches of mathematics, such as analysis and linear algebra the commutativity of well-known operations (such as addition and multiplication on real and complex numbers) is often used (or implicitly assumed ...
Switching {X,Y} in a graph. A two-graph is equivalent to a switching class of graphs and also to a (signed) switching class of signed complete graphs.. Switching a set of vertices in a (simple) graph means reversing the adjacencies of each pair of vertices, one in the set and the other not in the set: thus the edge set is changed so that an adjacent pair becomes nonadjacent and a nonadjacent ...
A set with a partial order on it is called a partially ordered set, poset, or just ordered set if the intended meaning is clear. By checking these properties, one immediately sees that the well-known orders on natural numbers , integers , rational numbers and reals are all orders in the above sense.
The algebra of sets is the set-theoretic analogue of the algebra of numbers. Just as arithmetic addition and multiplication are associative and commutative, so are set union and intersection; just as the arithmetic relation "less than or equal" is reflexive, antisymmetric and transitive, so is the set relation of "subset".