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Closure operators allow generalizing the concept of closure to any partially ordered set. Given a poset S whose partial order is denoted with ≤ , a closure operator on S is a function C : S → S {\displaystyle C:S\to S} that is
Convex hull (red) of a polygon (yellow). The usual set closure from topology is a closure operator. Other examples include the linear span of a subset of a vector space, the convex hull or affine hull of a subset of a vector space or the lower semicontinuous hull ¯ of a function : {}, where is e.g. a normed space, defined implicitly (¯) = ¯, where is the epigraph of a function .
In mathematics, particularly in functional analysis and topology, closed graph is a property of functions. [1] [2] A function f : X → Y between topological spaces has a closed graph if its graph is a closed subset of the product space X × Y. A related property is open graph. [3]
If : is a continuous function and is open, then is closed if and only if it converges to along every sequence converging to a boundary point of . [ 2 ] A closed proper convex function f is the pointwise supremum of the collection of all affine functions h such that h ≤ f (called the affine minorants of f ).
In mathematics, an expression or equation is in closed form if it is formed with constants, variables and a finite set of basic functions connected by arithmetic operations (+, −, ×, /, and integer powers) and function composition. Commonly, the allowed functions are nth root, exponential function, logarithm, and trigonometric functions.
The definition of a point of closure of a set is closely related to the definition of a limit point of a set.The difference between the two definitions is subtle but important – namely, in the definition of a limit point of a set , every neighbourhood of must contain a point of other than itself, i.e., each neighbourhood of obviously has but it also must have a point of that is not equal to ...
A linear operator : is closable in if there exists a vector subspace containing and a function (resp. multifunction) : whose graph is equal to the closure of the set in . Such an F {\displaystyle F} is called a closure of f {\displaystyle f} in X × Y {\displaystyle X\times Y} , is denoted by f ¯ , {\displaystyle {\overline {f}},} and ...
In mathematics, particularly abstract algebra, an algebraic closure of a field K is an algebraic extension of K that is algebraically closed. It is one of many closures in mathematics. Using Zorn's lemma [ 1 ] [ 2 ] [ 3 ] or the weaker ultrafilter lemma , [ 4 ] [ 5 ] it can be shown that every field has an algebraic closure , and that the ...