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In combinatorics, a branch of mathematics, partition regularity is one notion of largeness for a collection of sets.. Given a set , a collection of subsets is called partition regular if every set A in the collection has the property that, no matter how A is partitioned into finitely many subsets, at least one of the subsets will also belong to the collection.
The set of real numbers has several standard structures: An order: each number is either less than or greater than any other number. Algebraic structure: there are operations of addition and multiplication, the first of which makes it into a group and the pair of which together make it into a field .
For example, let C be the Smith–Volterra–Cantor set, and let I C be its indicator function. Because C is not Jordan measurable , I C is not Riemann integrable. Moreover, no function g equivalent to I C is Riemann integrable: g , like I C , must be zero on a dense set, so as in the previous example, any Riemann sum of g has a refinement ...
Partitions of a 4-element set ordered by refinement. A partition α of a set X is a refinement of a partition ρ of X—and we say that α is finer than ρ and that ρ is coarser than α—if every element of α is a subset of some element of ρ. Informally, this means that α is a further fragmentation of ρ. In that case, it is written that ...
In set theory, a Cartesian product is a mathematical operation which returns a set (or product set) from multiple sets. That is, for sets A and B, the Cartesian product A × B is the set of all ordered pairs (a, b) —where a ∈ A and b ∈ B. [5] The class of all things (of a given type) that have Cartesian products is called a Cartesian ...
(The article on unrestricted partition functions discusses this type of generating function.) For example, the coefficient of x 5 is +1 because there are two ways to split 5 into an even number of distinct parts (4 + 1 and 3 + 2), but only one way to do so for an odd number of distinct parts (the one-part partition 5).
The Cartesian square of a set X is the Cartesian product X 2 = X × X. An example is the 2-dimensional plane R 2 = R × R where R is the set of real numbers: [1] R 2 is the set of all points (x,y) where x and y are real numbers (see the Cartesian coordinate system).
In combinatorics, the rule of product or multiplication principle is a basic counting principle (a.k.a. the fundamental principle of counting). Stated simply, it is the intuitive idea that if there are a ways of doing something and b ways of doing another thing, then there are a · b ways of performing both actions.