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Two numbers, x and z, are said to be commutative under addition if

x + y = y + x

and to be commutative under multiplication if

x × y = xy × yx

In general, two elements a and b of a set S are commutative under the binary operation (an operation that works on two elements at a time) * if

a * b = ab * ba

Compare with associative and distributive.

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