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運算 ⟩ 二元運算 ⟩ 交換律 (commutativity)
若一個二元運算「 ∗{\color{orange}\ast}∗ 」擁有以下性質：
a∗b=b∗aa {\color{orange}\ast} b = b {\color{orange}\ast} aa∗b=b∗a
我們就說此運算具有「交換律」(commutativity)。
「Group╱群」的運算不一定具有「交換律」
Abstract and Linear Algebra (Burton) ⟩ 2.1 Definition and Elementary Properties of Groups (p. 45)
Understanding Analysis ⟩ 8.6 A Construction of R From Q, Def. 8.6.4 (p.299)