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An infimum of a set is always and only defined relative to a superset of the set in question. For example, there is no infimum of the positive real numbers inside the positive real numbers (as their own superset), nor any infimum of the positive real numbers inside the complex numbers with positive real part.
In mathematical analysis, limit superior and limit inferior are important tools for studying sequences of real numbers.Since the supremum and infimum of an unbounded set of real numbers may not exist (the reals are not a complete lattice), it is convenient to consider sequences in the affinely extended real number system: we add the positive and negative infinities to the real line to give the ...
For example, an infimum is just a categorical product. More generally, one can capture infima and suprema under the abstract notion of a categorical limit (or colimit , respectively). Another place where categorical ideas occur is the concept of a (monotone) Galois connection , which is just the same as a pair of adjoint functors .
For example, in lattice theory, one is interested in orders where all finite non-empty sets have both a least upper bound and a greatest lower bound. In domain theory, on the other hand, one focuses on partially ordered sets in which every directed subset has a supremum. Complete lattices and orders with a least element (the "empty supremum ...
The supremum of B is then equal to the infimum of X: since each element of X is an upper bound of B, sup B is smaller than all elements of X, i.e. sup B is in B. It is the greatest element of B and hence the infimum of X. In a dual way, the existence of all infima implies the existence of all suprema.
For example, if one takes the function () that is equal to zero everywhere except at = where () =, then the supremum of the function equals one. However, its essential supremum is zero since (under the Lebesgue measure ) one can ignore what the function does at the single point where f {\displaystyle f} is peculiar.
It is a common misconception that the limits infimum and supremum described here involve sets of accumulation points, that is, sets of =, where each is in some . This is only true if convergence is determined by the discrete metric (that is, x n → x {\displaystyle x_{n}\to x} if there is N {\displaystyle N} such that x n = x {\displaystyle x ...
An example is given by the power set of a set, partially ordered by inclusion, for which the supremum is the union and the infimum is the intersection. Another example is given by the natural numbers, partially ordered by divisibility, for which the supremum is the least common multiple and the infimum is the greatest common divisor.