Deaths_ally wrote:trythil wrote:
A and B are equivalent if and only if A is a subset of B and B is a subset of A.
At least that's the definition I learned in discrete math. Linguists seem to define sets A, B equivalent iff they contain the same number of elements, which isn't the same thing.
*head spins.. passes out*
It's pretty simple, actually.
Take sets A, B from the set of all positive integers:
A = {1, 2, 3, 4, 5}
B = {1, 2, 3, 4, 5}
Is A a subset of B? Yes, A is a subset of B. (It's not a
proper subset of B, since A
is B, but a subset can include the entire set.)
Is B a subset of A? Again, yes. So A = B, and B = A.
Let's try this again:
A = {1, 2, 3}
B = {1, 2, 3, 4, 5}
Is A a subset of B? Yes, because A is {1, 2, 3}, and B is {1, 2, 3, 4, 5}.
Is B a subset of A? No, because B contains elements that are not in A -- namely, 4 and 5. So A is not equal to B, and B is not equal to A.
This isn't really a proof of the statement "sets A and B are equal to each other if A is a subset of B and B is a subset of A", but it does demonstrate how the statement works.