In mathematics, the upper topology on a partially ordered set X is the coarsest topology in which the closure of a singleton is the order section for each If is a partial order, the upper topology is the least order consistent topology in which all open sets are up-sets. However, not all up-sets must necessarily be open sets. The lower topology induced by the preorder is defined similarly in terms of the down-sets. The preorder inducing the upper topology is its specialization preorder, but the specialization preorder of the lower topology is opposite to the inducing preorder.
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The real upper topology is most naturally defined on the upper-extended real line by the system of open sets. Similarly, the real lower topology is naturally defined on the lower real line A real function on a topological space is upper semi-continuous if and only if it is lower-continuous, i.e. is continuous with respect to the lower topology on the lower-extended line Similarly, a function into the upper real line is lower semi-continuous if and only if it is upper-continuous, i.e. is continuous with respect to the upper topology on
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See also
[edit]- List of topologies – List of concrete topologies and topological spaces
References
[edit]- Gerhard Gierz; K.H. Hofmann; K. Keimel; J. D. Lawson; M. Mislove; D. S. Scott (2003). Continuous Lattices and Domains. Cambridge University Press. p. 510. ISBN 0-521-80338-1.
- Kelley, John L. (1955). General Topology. Van Nostrand Reinhold. p. 101.
- Knapp, Anthony W. (2005). Basic Real Analysis. Birkhhauser. p. 481. ISBN 0-8176-3250-6.
Well, that’s interesting to know that Psilotum nudum are known as whisk ferns. Psilotum nudum is the commoner species of the two. While the P. flaccidum is a rare species and is found in the tropical islands. Both the species are usually epiphytic in habit and grow upon tree ferns. These species may also be terrestrial and grow in humus or in the crevices of the rocks.
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