Infimum

Definition

The infimum, often referred to as the greatest lower bound, is the largest value that is less than or equal to all elements in a given subset of a partially ordered set. Understanding the infimum is crucial because it connects to concepts like completeness in lattices, where every subset should have an infimum within the structure.

5 Must Know Facts For Your Next Test

1. An infimum may or may not be an element of the subset it bounds; however, if it exists in the subset, it serves as both the infimum and the minimum.
2. In complete lattices, every subset has both an infimum and a supremum, ensuring that both bounds are always attainable.
3. When dealing with chains (totally ordered sets), the infimum can be easily found as long as the subset has a lower bound.
4. The existence of an infimum for subsets is essential for establishing completeness in mathematical structures and plays a role in fixed point theorems.
5. In terms of duality, while the infimum relates to lower bounds, its counterpart, the supremum, deals with upper bounds, illustrating a balance within order theory.

Review Questions

• How does the concept of infimum relate to completeness in lattices?
  • The concept of infimum is integral to completeness in lattices because completeness requires that every subset has both an infimum and a supremum. This means that for any subset of a complete lattice, one can always identify its greatest lower bound, ensuring that there are no gaps in the ordering. This property allows for effective analysis and application of various mathematical principles within these structures.
• Compare and contrast the infimum and supremum in terms of their roles within partially ordered sets.
  • The infimum and supremum serve as essential boundaries within partially ordered sets. The infimum represents the greatest lower bound, while the supremum denotes the least upper bound. Their roles are complementary: while the infimum identifies the largest value below all elements in a subset, the supremum identifies the smallest value above them. This duality enhances our understanding of order relations and their implications in various mathematical contexts.
• Evaluate the significance of the infimum when considering Dedekind-MacNeille completion of posets.
  • The significance of the infimum in Dedekind-MacNeille completion lies in its role in ensuring that every subset within a poset can be fully bounded. The completion process aims to create a complete lattice by including necessary suprema and infima for all subsets. By adding these bounds, one can resolve any ambiguities about order relations and achieve a refined structure where every element behaves predictably with respect to its bounds, thus facilitating deeper analysis and applications in order theory.

Related terms

Supremum: The supremum, or least upper bound, is the smallest value that is greater than or equal to all elements in a subset of a partially ordered set.

Greatest Element: A greatest element in a set is an element that is greater than or equal to every other element in that set.

Poset: A partially ordered set (poset) is a set combined with a relation that describes how its elements are ordered relative to each other.