If a is an upper bound for A, then sup(A) exists, and:
sup(A) <= a
If a is an element of A, then, since sup(A) is an upper bound on A, we have:
a <= sup(A)
Thus, sup(A) = a.
sup(A) <= a
If a is an element of A, then, since sup(A) is an upper bound on A, we have:
a <= sup(A)
Thus, sup(A) = a.