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・ Extinction paradox
・ Extinction probability
・ Extinction ratio
・ Extinction risk from global warming
・ Extinction threshold
・ Extinction vortex
・ Extinctioners
・ Extinctionism
・ Extinguishment
・ Extirpation
・ Extize
・ EXTL1
・ EXTL2
・ EXTL3
・ External (disambiguation)
External (mathematics)
・ External Active Thermal Control System
・ External Affairs (film)
・ External anal sphincter
・ External arcuate fibers
・ External association
・ External auditor
・ External bacterial infection (fish)
・ External ballistics
・ External beam radiotherapy
・ External border of the European Union
・ External Bus Interface
・ External Bus Interface 2
・ External capsule
・ External cardinal


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External (mathematics) : ウィキペディア英語版
External (mathematics)

The term external is useful for describing certain algebraic structures. The term comes from the concept of an external binary operation which is a binary operation that draws from some ''external set''. To be more specific, a left external binary operation on ''S'' over ''R'' is a function f : R \times S \rightarrow S and a right external binary operation on ''S'' over ''R'' is a function f : S \times R \rightarrow S where ''S'' is the set the operation is defined on, and ''R'' is the external set (the set the operation is defined ''over'').
== Generalizations ==

The ''external'' concept is a generalization rather than a specialization, and as such, it is different from many terms in mathematics. A similar but opposite concept is that of an ''internal binary function'' from ''R'' to ''S'', defined as a function f : R \times R \rightarrow S. Internal binary functions are like binary functions, but are a form of specialization, so they only accept a subset of the domains of binary functions. Here we list these terms with the function signatures they imply, along with some examples:
* f : Q \times R \rightarrow S (binary function)
*
* Example: exponentiation (z^q : \Bbb \times \Bbb \rightarrow \Bbb as in ^ = i),
*
* Example: set membership ((\in) : S \times \mathbf \rightarrow \Bbb where \mathbf is the category of sets)
*
* Examples: matrix multiplication, the tensor product, and the Cartesian product
* f : R \times R \rightarrow S (internal binary function)
*
* Example: internal binary relations ((\le) : R \times R \rightarrow \Bbb)
*
* Examples: the dot product, the inner product, and metrics.
* f : R \times S \rightarrow S (external binary operation)
*
* Examples: dynamical system flows, group actions, projection maps, and scalar multiplication.
* f : S \times S \rightarrow S (binary operation).
*
* Examples: addition, multiplication, permutations, and the cross product.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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