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恒等函数.
恒等函数(英语:Identity function)是数学中对于传回和其输入值相同的函数的称呼。换句话说,恒等函数为函数
。
定义
设M为一集合,于M上的恒等函数f被定义于一具有定义域和陪域M的函数,其对任一M内的元素x,会有
的关系。
于M上的恒等函数f通常标记为
或
。
代数性质
设f : M → N为任一函数,则会有f o idM = f = idN o f(其中"o"为函数复合)。特别地是,idM会是所有由M至M的函数所组成之幺半群的单位元。
因为幺半群的单位元是唯一的,也可以反过来把M上的恒等函数定义为这个幺半群的单位元。此一定义广义化成了于范畴论中恒等态射的概念,其中M的自同态并不必然是函数。
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