The Unapologetic Mathematician

Mathematics for the interested outsider

Orders

As a bonus today, I want to define a few more kinds of relations.

A preorder is a relation on a set which is reflexive and transitive. We often write a general preorder as x\preceq y and say that x precedes y or that y succeeds x. A set equipped with a preorder is called a preordered set. If we also have that for any two elements x and y there is some element z (possibly the same as x or y) that succeeds both of them we call the structure a directed set.

A partial order is a preorder which is also antisymmetric: the only way to have both x\preceq y and y\preceq x is for x and y to be the same element. We call a set with a partial order a partially-ordered set or a “poset”.

Any set gives a partial order on its set of subsets, given by inclusion: if A and B are subsets of a set X, then A precedes B if A is contained in B. This has the further nice property that it has a top element, X itself, that succeeds every element. It also has a bottom element, the empty subset, that precedes everything. The same sort of construction applies to give the poset of subgroups of any given group. These kinds of partially-ordered sets are very important in logic and set theory, and they’ll come up in more detail later.

Finally, a partial order where for any two elements x and y we either have x\preceq y or y\preceq x is called a total order. Total orders show up over and over, and they’re nice things to have around. I must admit, though, that as far as I’m concerned they’re pretty boring in and of themselves.

March 11, 2007 - Posted by John Armstrong | Fundamentals, Orders | | 6 Comments

6 Comments »

  1. [...] A well-ordering on a set is a special kind of total order: one in which every non-empty subset contains a least [...]

    Pingback by Well-Ordering « The Unapologetic Mathematician | April 2, 2007

  2. [...] Lower Bounds and Euclid’s Algorithm One interesting question for any partial order is that of lower or upper bounds. Given a partial order and a subset we say that is a lower [...]

    Pingback by Greatest Lower Bounds and Euclid's Algorithm « The Unapologetic Mathematician | May 4, 2007

  3. [...] A poset which has both least upper bounds and greatest lower bounds is called a lattice. In more detail, [...]

    Pingback by Lattices « The Unapologetic Mathematician | May 14, 2007

  4. [...] containment from to those collections of subsets of which are actually topologies, it defines a partial order on the collection of all topologies on [...]

    Pingback by Topology « The Unapologetic Mathematician | November 5, 2007

  5. [...] numbers for sequences is that they’re “directed”. That is, there’s an order on them. It’s a particularly simple order since it’s total — any two elements are [...]

    Pingback by Nets, Part I « The Unapologetic Mathematician | November 19, 2007

  6. [...] let’s consider the collection of all subspaces of . This is a partially-ordered set, where the order is given by containment of the underlying sets. It’s sort of like the power [...]

    Pingback by The Sum of Subspaces « The Unapologetic Mathematician | July 21, 2008

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