Is a Finite Lattice also a Complete Lattice?

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Discussion Overview

The discussion centers on whether a finite lattice is also a complete lattice, exploring definitions and implications within the context of lattice theory.

Discussion Character

  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions if a finite lattice is automatically a complete lattice and seeks clarification on the reasoning behind this.
  • Another participant asserts that it is true, explaining that a lattice requires every finite subset to have a supremum and infimum, while a complete lattice requires this for every subset.
  • A similar viewpoint is reiterated, emphasizing that since every subset of a finite set is also finite, it follows that every subset has a supremum and infimum.
  • A later reply introduces the idea that the definition of a lattice may vary, noting that some definitions only require the supremum and infimum for pairs of elements, suggesting that proof by induction may be necessary in some contexts.

Areas of Agreement / Disagreement

Participants generally agree on the definitions of lattice and complete lattice, but there is some uncertainty regarding the necessity of proof by induction and the implications of different definitions.

Contextual Notes

The discussion highlights the dependence on definitions used in different contexts, particularly regarding the requirements for a set to be classified as a lattice or complete lattice.

XodoX
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I'm not sure if I am using the right terms here, but:

When X is a finite set and R is a relation...

If (X,R) is a lattice, then (X,R) is also a complete lattice.



Does this make sense? The question then is, why is is also automatically complete. I don't understand that.
 
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Yes, it makes sense and it is true.

A lattice is an ordered set where every finite subset has a supremum and an infimum. A complete lattice is an ordered set where every subset has a supremum and an infimum.

Clearly, if the underlying set ##X## is finite, then every subset of ##X## is finite. Thus every subset has a supremum and an infimum since every finite subset has a supremum and an infimum.
 
micromass said:
Yes, it makes sense and it is true.

A lattice is an ordered set where every finite subset has a supremum and an infimum. A complete lattice is an ordered set where every subset has a supremum and an infimum.

Clearly, if the underlying set ##X## is finite, then every subset of ##X## is finite. Thus every subset has a supremum and an infimum since every finite subset has a supremum and an infimum.

Makes sense. Does this prove it, though ? I mean, is there any calculation to prove this, or you just basically say that's how it is.
 
It depends on the definition you've used. Some books only require, for a poset to be a lattice, that every pair ##x,y## of elements have a supremum ##x\vee y:= \bigvee\{x,y\}## and infimum ##x\wedge y:= \bigwedge\{x,y\}##. If this is so in your course, you still need to prove (by induction) that every finite set in a lattice has a supremum and infimum.
 
Thank you! Didn't think of proof by induction.
 

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