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In mathematics, a cancellative semigroup (also called a cancellation semigroup) is a semigroup having the cancellation property. In intuitive terms, the cancellation property asserts that from an equality of the form ''a'' · ''b'' = ''a'' · ''c'', where · is a binary operation, one can cancel the element ''a'' and deduce the equality ''b'' = ''c''. In this case the element being canceled out is appearing as the left factors of ''a'' · ''b'' and ''a'' · ''c'' and hence it is a case of left cancellation property. The right cancellation property can be defined in an analogous way. Prototypical examples of cancellative semigroups are the groups and the semigroup of positive integers under addition or multiplication. Cancellative semigroups are considered to be very close to being groups because cancellability is one of the necessary conditions for a semigroup to be embeddable in a group. Moreover, every finite cancellative semigroup is a group. One of the main problems associated with the study of cancellative semigroups is to determine the necessary and sufficient conditions for embedding a cancellative semigroup in a group. The origins of the study of cancellative semigroups can be traced to the first substantial paper on semigroups, . ==Formal definitions== Let ''S'' be a semigroup. An element ''a'' in ''S'' is left cancellative (or, is 'left cancellable', or, has the 'left cancellation property') if ''ab'' = ''ac'' implies ''b'' = ''c'' for all ''b'' and ''c'' in ''S''. If every element in ''S'' is left cancellative, then ''S'' is called a left cancellative semigroup. Let ''S'' be a semigroup. An element ''a'' in ''S'' is right cancellative (or, is 'right cancellable', or, has the 'right cancellation property') if ''ba'' = ''ca'' implies ''b'' = ''c'' for all ''b'' and ''c'' in ''S''. If every element in ''S'' is right cancellative, then ''S'' is called a right cancellative semigroup. Let ''S'' be a semigroup. If every element in ''S'' is both left cancellative and right cancellative, then ''S'' is called a cancellative semigroup. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「cancellative semigroup」の詳細全文を読む スポンサード リンク
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