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and that the equality holds if and only if the curve is a circle. There are, in fact, two ways to measure the spherical area enclosed by a simple closed curve, but the inequality is symmetric with the respect to taking the complement. This inequality was discovered by Paul Lévy (1919) who also extended it to higher dimensions and general ...
An interval is said to be left-open if and only if it contains no minimum (an element that is smaller than all other elements); right-open if it contains no maximum; and open if it contains neither. The interval [0, 1) = {x | 0 ≤ x < 1}, for example, is left-closed and right-open. The empty set and the set of all reals are both open and ...
In geometry, a disk (also spelled disc) [1] is the region in a plane bounded by a circle. A disk is said to be closed if it contains the circle that constitutes its boundary, and open if it does not. [2] For a radius, , an open disk is usually denoted as and a closed disk is ¯.
This function from the unit circle to the half-open interval [0,2π) is bijective, open, and closed, but not continuous. It shows that the image of a compact space under an open or closed map need not be compact. Also note that if we consider this as a function from the unit circle to the real numbers, then it is neither open nor closed.
A closed half-space is a set in the form {()} or {()}, and likewise an open half-space uses strict inequality. [11] [12] Half-spaces (open or closed) are affine convex cones. Moreover (in finite dimensions), any convex cone C that is not the whole space V must be contained in some closed half-space H of V; this is a special case of Farkas' lemma.
The open mapping theorem points to the sharp difference between holomorphy and real-differentiability. On the real line , for example, the differentiable function f ( x ) = x 2 {\displaystyle f(x)=x^{2}} is not an open map, as the image of the open interval ( − 1 , 1 ) {\displaystyle (-1,1)} is the half-open interval [ 0 , 1 ) {\displaystyle ...
In Euclidean space of dimensions, the ()-dimensional unit sphere is the set of all points (, …,) which satisfy the equation + + + = The open unit -ball is the set of all points satisfying the inequality
A set is clopen if it is both open and closed. Closed ball If (M, d) is a metric space, a closed ball is a set of the form D(x; r) := {y in M : d(x, y) ≤ r}, where x is in M and r is a positive real number, the radius of the ball. A closed ball of radius r is a closed r-ball. Every closed ball is a closed set in the topology induced on M by d.