In mathematics, A does not mean "different from A" and is usually used in logical propositions. Refers to the negation of A, including the negative and negative sides of A. For example, in a set, A does not exist in the set, so A is not another element in the set.

A non can also be understood as "except A". In mathematics, we often encounter the situation that A is not B, that is to say, all elements except A should be B under certain conditions.

"Not" is also often used in mathematics to exclude certain situations. For example, in a theorem, when certain conditions are met, it can be said that "the theorem still holds for the case that A is not B". This means that even if the case that A is B is not satisfied, the theorem is still applicable to other cases.