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This concept is often contrasted with uniform convergence.To say that = means that {| () |:} =, where is the common domain of and , and stands for the supremum.That is a stronger statement than the assertion of pointwise convergence: every uniformly convergent sequence is pointwise convergent, to the same limiting function, but some pointwise convergent sequences are not uniformly convergent.
The most basic type of convergence for a sequence of functions (in particular, it does not assume any topological structure on the domain of the functions) is pointwise convergence. It is defined as convergence of the sequence of values of the functions at every point.
A net = is said to be frequently or cofinally in if for every there exists some such that and . [5] A point is said to be an accumulation point or cluster point of a net if for every neighborhood of , the net is frequently/cofinally in . [5] In fact, is a cluster point if and only if it has a subnet that converges to . [6] The set of all ...
This definition is technically called Q-convergence, short for quotient-convergence, and the rates and orders are called rates and orders of Q-convergence when that technical specificity is needed. § R-convergence , below, is an appropriate alternative when this limit does not exist.
The radius of convergence of a power series f centered on a point a is equal to the distance from a to the nearest point where f cannot be defined in a way that makes it holomorphic. The set of all points whose distance to a is strictly less than the radius of convergence is called the disk of convergence.
Convergence spaces generalize the notions of convergence that are found in point-set topology, including metric convergence and uniform convergence. Every topological space gives rise to a canonical convergence but there are convergences, known as non-topological convergences, that do not arise from any topological space. [1]
This definition of a cluster or accumulation point of a sequence generalizes to nets and filters. The similarly named notion of a limit point of a sequence [1] (respectively, a limit point of a filter, [2] a limit point of a net) by definition refers to a point that the sequence converges to (respectively, the filter converges to, the net ...
A sequence of functions () converges uniformly to when for arbitrary small there is an index such that the graph of is in the -tube around f whenever . The limit of a sequence of continuous functions does not have to be continuous: the sequence of functions () = (marked in green and blue) converges pointwise over the entire domain, but the limit function is discontinuous (marked in red).