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pointwise (Definition)

When concepts (properties, operations, etc.) on a set $Y$ are extended to functions $f\colon X \longrightarrow Y$ by treating each function value $f(x)$ in isolation, the extended concept is often qualified with the word pointwise. One example is pointwise convergence of functions--a sequence $\{f_n\}_{n=1}^\infty$ of functions $X \longrightarrow Y$ converges pointwise to a function $f$ if $ \lim_{n \rightarrow \infty} f_n(x) = f(x)$ for all $ x \in X$.

An important class of pointwise concepts are the pointwise operations--operations defined on functions by applying the operations to function values separately for each point in the domain of definition. These include

$\displaystyle (f+g)(x) ={}$ $\displaystyle f(x)+g(x)$   (pointwise addition)    
$\displaystyle (f \cdot g)(x) ={}$ $\displaystyle f(x) \cdot g(x)$   (pointwise multiplication)    
$\displaystyle (\lambda f)(x) ={}$ $\displaystyle \lambda \cdot f(x)$   (pointwise multiplication by scalar)    

where the identities hold for all $ x \in X$. Pointwise operations inherit such properties as associativity, commutativity, and distributivity from corresponding operations on $Y$ .

An example of an operation on functions which is not pointwise is the convolution product.




"pointwise" is owned by lars_h.
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Also defines:  pointwise operation, pointwise addition, pointwise muliplication
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Cross-references: product, distributivity, commutativity, associativity, identities, domain of definition, point, converges, sequence, pointwise convergence, functions, operations, properties
There are 34 references to this entry.

This is version 1 of pointwise, born on 2005-07-25.
Object id is 7260, canonical name is Pointwise2.
Accessed 6759 times total.

Classification:
AMS MSC03-00 (Mathematical logic and foundations :: General reference works )
 08-00 (General algebraic systems :: General reference works )

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