Identity Element
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An identity element is that element of a set of numbers which, when combined with another number in a particular operation, leaves that number unchanged.
Zero (0) is the identity element under addition for the real numbers. If a is any real number, a + 0 = a and 0 + a = a.
One (1) is the identity element under multiplication since a à 1 = a and 1 à a = a.