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13934 and other numbers x such that x ≥ 13934 would be an upper bound for S. The set S = {42} has 42 as both an upper bound and a lower bound; all other numbers are either an upper bound or a lower bound for that S. Every subset of the natural numbers has a lower bound since the natural numbers have a least element (0 or 1, depending on ...
Then has an upper bound (, for example, or ) but no least upper bound in : If we suppose is the least upper bound, a contradiction is immediately deduced because between any two reals and (including and ) there exists some rational , which itself would have to be the least upper bound (if >) or a member of greater than (if <).
By the least-upper-bound property, S has a least upper bound c ∈ [a, b]. Hence, c is itself an element of some open set U α, and it follows for c < b that [a, c + δ] can be covered by finitely many U α for some sufficiently small δ > 0. This proves that c + δ ∈ S and c is not an upper bound for S. Consequently, c = b.
The element k is called an upper bound of S. The concepts of bounded below and lower bound are defined similarly. (See also upper and lower bounds.) A subset S of a partially ordered set P is called bounded if it has both an upper and a lower bound, or equivalently, if it is contained in an interval.
Thus, the infimum or meet of a collection of subsets is the greatest lower bound while the supremum or join is the least upper bound. In this context, the inner limit, lim inf X n, is the largest meeting of tails of the sequence, and the outer limit, lim sup X n, is the smallest joining of tails of the sequence. The following makes this precise.
However, this set has no least upper bound in Q: the least upper bound as a subset of the reals would be √2, but it does not exist in Q. For any upper bound x ∈ Q, there is another upper bound y ∈ Q with y < x. For instance, take x = 1.5, then x is certainly an upper bound of S, since x is positive and x 2 = 2.25 ≥ 2; that is, no ...
In a directed set, every pair of elements (particularly pairs of incomparable elements) has a common upper bound within the set. If a directed set has a maximal element, it is also its greatest element, [proof 7] and hence its only maximal element. For a directed set without maximal or greatest elements, see examples 1 and 2 above.
Graham's number is an immense number that arose as an upper bound on the answer of a problem in the mathematical field of Ramsey theory.It is much larger than many other large numbers such as Skewes's number and Moser's number, both of which are in turn much larger than a googolplex.