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bounded function (Definition)

Definition Suppose $ X$ is a nonempty set. Then a function $ f:X\to \mathbb{C}$ is a bounded function if there exist a $ C<\infty$ such that $ \vert f(x)\vert<C$ for all $ x\in X$. The set of all bounded functions on $ X$ is usually denoted by $ B(X)$ ([1], pp. 61).

Under standard point-wise addition and point-wise multiplication by a scalar, $ B(X)$ is a complex vector space.

If $ f\in B(X)$, then the sup-norm, or uniform norm, of $ f$ is defined as

$\displaystyle \vert\vert f\vert\vert _\infty = \sup_{x\in X} \vert f(x)\vert. $
It is straightforward to check that $ \vert\vert\cdot\vert\vert _\infty$ makes $ B(X)$ into a normed vector space, i.e., to check that $ \vert\vert\cdot\vert\vert _\infty$ satisfies the assumptions for a norm.

Example

Suppose $ X$ is a compact topological space. Further, let $ C(X)$ be the set of continuous complex-valued functions on $ X$ (with the same vector space structure as $ B(X)$). Then $ C(X)$ is a vector subspace of $ B(X)$.

References

1
C.D. Aliprantis, O. Burkinshaw, Principles of Real Analysis, 2nd ed., Academic Press, 1990.



"bounded function" is owned by Koro. [ full author list (2) | owner history (1) ]
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Also defines:  supremum norm, sup norm, sup-norm, uniform norm, bounded function, unbounded function
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Cross-references: vector subspace, structure, continuous, topological space, compact, norm, normed vector space, vector space, complex, scalar, multiplication, addition, function
There are 28 references to this entry.

This is version 4 of bounded function, born on 2003-07-06, modified 2004-12-11.
Object id is 4426, canonical name is BoundedFunction.
Accessed 21140 times total.

Classification:
AMS MSC46-00 (Functional analysis :: General reference works )

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