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Closed graph theorem [5] — If : is a map from a topological space into a Hausdorff space, then the graph of is closed if : is continuous. The converse is true when Y {\displaystyle Y} is compact .
Function with a sequentially closed graph. If f : X → Y is a function then the following are equivalent: f has a sequentially closed graph (in X × Y); (definition) the graph of f is a sequentially closed subset of X × Y; for every x ∈ X and sequence x • = (x i) ∞ i=1 in X such that x • → x in X, if y ∈ Y is such that the net f(x ...
In practice, this works like this: T is some operator on some function space. One shows T is continuous with respect to the distribution topology ; thus, the graph is closed in that topology, which implies closedness in the norm topology and then T is a bounded by the closed graph theorem (when the theorem applies).
Given a function: from a set X (the domain) to a set Y (the codomain), the graph of the function is the set [4] = {(, ()):}, which is a subset of the Cartesian product.In the definition of a function in terms of set theory, it is common to identify a function with its graph, although, formally, a function is formed by the triple consisting of its domain, its codomain and its graph.
In this case, Y is the set of real numbers R with the standard metric d Y (y 1, y 2) = |y 1 − y 2 |, and X is a subset of R. In general, the inequality is (trivially) satisfied if x 1 = x 2. Otherwise, one can equivalently define a function to be Lipschitz continuous if and only if there exists a constant K ≥ 0 such that, for all x 1 ≠ x 2,
For functions that are not uniformly continuous, this isn't possible; for these functions, the graph might lie inside the height of the rectangle at some point on the graph but there is a point on the graph where the graph lies above or below the rectangle. (the graph penetrates the top or bottom side of the rectangle.)
No such function exists around point B. However at B it is possible to write a function x(y) that describes the solution set locally. If we define the function f(x, y) = x 2 + y 2, then the equation f(x, y) = 1 cuts out the unit circle as the level set {(x, y) | f(x, y) = 1}. There is no way to represent the unit circle as the graph of a ...
These identities are useful whenever expressions involving trigonometric functions need to be simplified. An important application is the integration of non-trigonometric functions: a common technique involves first using the substitution rule with a trigonometric function, and then simplifying the resulting integral with a trigonometric identity.