Supremum

Definition

The supremum of a set is the least upper bound of that set, which means it is the smallest number that is greater than or equal to every element in the set. It’s a crucial concept in the context of partial orders and lattices, as it helps in understanding how elements relate to one another within ordered structures. The supremum exists for subsets of a partially ordered set and can also serve as a point of reference when discussing the completeness properties of lattices.

5 Must Know Facts For Your Next Test

1. Not every subset has a supremum within its parent set, but when it does exist, it is unique.
2. The supremum of a finite set can be found easily by identifying the maximum element.
3. For bounded sets, the supremum will always be an upper bound for the set.
4. In real numbers, if a set is not empty and has an upper bound, the supremum will always exist due to the completeness property of real numbers.
5. The concept of supremum extends beyond numbers; it can apply to functions, sequences, and other mathematical objects when considering their bounds.

Review Questions

• How does the concept of supremum differ from maximum in a partially ordered set?
  • The supremum refers to the least upper bound of a set, which may not necessarily be an element of that set, while the maximum is the largest actual element in the set. A set can have a supremum even if it lacks a maximum; for instance, in the open interval (0, 1), the supremum is 1, which is not contained in the set itself. Understanding this distinction helps clarify the bounds and relationships within partially ordered sets.
• Discuss how the existence of a supremum influences the structure of a complete lattice.
  • In a complete lattice, every subset must have both a supremum and an infimum, which guarantees a well-defined structure where all elements are properly ordered. This property allows for consistency in defining operations like joins (supremums) and meets (infimums) across various subsets. Without this existence guarantee, the lattice could become less coherent and complicate mathematical reasoning about its elements.
• Evaluate how the concept of supremum applies to functions defined over intervals and what implications this has for analysis.
  • When dealing with functions over intervals, the concept of supremum becomes vital for understanding their behavior, particularly regarding limits and continuity. For instance, if you take a continuous function on a closed interval, its supremum provides insight into its maximum output value within that range. This concept plays a crucial role in calculus and real analysis, helping to establish foundational results like the Extreme Value Theorem, which asserts that continuous functions achieve their suprema and infima on closed intervals.

Related terms

Infimum: The infimum of a set is the greatest lower bound, representing the largest number that is less than or equal to every element in the set.

Upper Bound: An upper bound for a set is any number that is greater than or equal to every element in that set.

Complete Lattice: A complete lattice is a partially ordered set in which every subset has both a supremum and an infimum.