Mathematics

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What does it mean in mathematics for two things to be equal, or for two things to have the same "identity"?
For example, because anything divided by zero is "undefined", can we say that 1/0 = 2/0?
What about the relational database concept of "null" which is supposed to stand for "unknown"? In relational algebra, they say NULL is not equal to NULL, but doesn't that violate the law of identity that everything is equal to itself?
Accepted:July 11, 2007

Accepted:

July 11, 2007