Well-quasi-ordering

Transitive binary relations

	Symmetric	Antisymmetric	Connected	Well-founded	Has joins	Has meets	Reflexive	Irreflexive	Asymmetric
			Total, Semiconnex					Anti- reflexive
Equivalence relation	Y	✗	✗	✗	✗	✗	Y	✗	✗
Preorder (Quasiorder)	✗	✗	✗	✗	✗	✗	Y	✗	✗
Partial order	✗	Y	✗	✗	✗	✗	Y	✗	✗
Total preorder	✗	✗	Y	✗	✗	✗	Y	✗	✗
Total order	✗	Y	Y	✗	✗	✗	Y	✗	✗
Prewellordering	✗	✗	Y	Y	✗	✗	Y	✗	✗
Well-quasi-ordering	✗	✗	✗	Y	✗	✗	Y	✗	✗
Well-ordering	✗	Y	Y	Y	✗	✗	Y	✗	✗
Lattice	✗	Y	✗	✗	Y	Y	Y	✗	✗
Join-semilattice	✗	Y	✗	✗	Y	✗	Y	✗	✗
Meet-semilattice	✗	Y	✗	✗	✗	Y	Y	✗	✗
Strict partial order	✗	Y	✗	✗	✗	✗	✗	Y	Y
Strict weak order	✗	Y	✗	✗	✗	✗	✗	Y	Y
Strict total order	✗	Y	Y	✗	✗	✗	✗	Y	Y
	Symmetric	Antisymmetric	Connected	Well-founded	Has joins	Has meets	Reflexive	Irreflexive	Asymmetric
Definitions, for all $a, b$ and $S \neq \emptyset :$	$\begin{matrix} a R b \\ \Rightarrow & b R a \end{matrix}$	$\begin{matrix} a R b and & b R a \\ \Rightarrow a = & b \end{matrix}$	$\begin{matrix} a \neq & b \Rightarrow \\ a R b or & b R a \end{matrix}$	$\begin{matrix} min S \\ exists \end{matrix}$	$\begin{matrix} a \lor b \\ exists \end{matrix}$	$\begin{matrix} a \land b \\ exists \end{matrix}$	$a R a$	$not a R a$	$\begin{matrix} a R b \Rightarrow \\ not b R a \end{matrix}$

indicates that the column's property is always true for the row's term (at the very left), while ✗ indicates that the property is not guaranteed
in general (it might, or might not, hold). For example, that every equivalence relation is symmetric, but not necessarily antisymmetric,
is indicated by

in the "Symmetric" column and ✗ in the "Antisymmetric" column, respectively.

All definitions tacitly require the homogeneous relation $R$ be transitive: for all $a, b, c,$ if $a R b$ and $b R c$ then $a R c .$
A term's definition may require additional properties that are not listed in this table.

In mathematics, specifically order theory, a well-quasi-ordering or wqo on a set $X$ is a quasi-ordering of $X$ for which every infinite sequence of elements $x_{0}, x_{1}, x_{2}, \dots$ from $X$ contains a non-decreasing pair $x_{i} \leq x_{j}$ with $i < j .$

Motivation

Well-founded induction can be used on any set with a well-founded relation, thus one is interested in when a quasi-order is well-founded. (Here, by abuse of terminology, a quasiorder $\leq$ is said to be well-founded if the corresponding strict order $x \leq y \land y ≰ x$ is a well-founded relation.) However the class of well-founded quasiorders is not closed under certain operations—that is, when a quasi-order is used to obtain a new quasi-order on a set of structures derived from our original set, this quasiorder is found to be not well-founded. By placing stronger restrictions on the original well-founded quasiordering one can hope to ensure that our derived quasiorderings are still well-founded.

An example of this is the power set operation. Given a quasiordering $\leq$ for a set $X$ one can define a quasiorder $\leq^{+}$ on $X$ 's power set $P (X)$ by setting $A \leq^{+} B$ if and only if for each element of $A$ one can find some element of $B$ that is larger than it with respect to $\leq$ . One can show that this quasiordering on $P (X)$ need not be well-founded, but if one takes the original quasi-ordering to be a well-quasi-ordering, then it is.

Formal definition

A well-quasi-ordering on a set $X$ is a quasi-ordering (i.e., a reflexive, transitive binary relation) such that any infinite sequence of elements $x_{0}, x_{1}, x_{2}, \dots$ from $X$ contains an increasing pair $x_{i} \leq x_{j}$ with $i < j$ . The set $X$ is said to be well-quasi-ordered, or shortly wqo.

A well partial order, or a wpo, is a wqo that is a proper ordering relation, i.e., it is antisymmetric.

Among other ways of defining wqo's, one is to say that they are quasi-orderings which do not contain infinite strictly decreasing sequences (of the form $x_{0} > x_{1} > x_{2} > \dots$ ) nor infinite sequences of pairwise incomparable elements. Hence a quasi-order (X, ≤) is wqo if and only if (X, <) is well-founded and has no infinite antichains.

Ordinal type

Let $X$ be well partially ordered. A (necessarily finite) sequence $(x_{1}, x_{2}, \dots, x_{n})$ of elements of $X$ that contains no pair $x_{i} \leq x_{j}$ with $i < j$ is usually called a bad sequence. The tree of bad sequences $T_{X}$ is the tree that contains a vertex for each bad sequence, and an edge joining each nonempty bad sequence $(x_{1}, \dots, x_{n - 1}, x_{n})$ to its parent $(x_{1}, \dots, x_{n - 1})$ . The root of $T_{X}$ corresponds to the empty sequence. Since $X$ contains no infinite bad sequence, the tree $T_{X}$ contains no infinite path starting at the root. Therefore, each vertex $v$ of $T_{X}$ has an ordinal height $o (v)$ , which is defined by transfinite induction as $o (v) = lim_{w c h i l d o f v} (o (w) + 1)$ . The ordinal type of $X$ , denoted $o (X)$ , is the ordinal height of the root of $T_{X}$ .

A linearization of $X$ is an extension of the partial order into a total order. It is easy to verify that $o (X)$ is an upper bound on the ordinal type of every linearization of $X$ . De Jongh and Parikh proved that in fact there always exists a linearization of $X$ that achieves the maximal ordinal type $o (X)$ .

Examples

Pic.1: A non-example: integers with the usual order

$(ℕ, \leq)$ , the set of natural numbers with standard ordering, is a well partial order (in fact, a well-order). However, $(ℤ, \leq)$ , the set of positive and negative integers (see Pic.1), is not a well-quasi-order, because it is not well-founded. The infinite sequence -1, -2,... contains no increasing pair.
$(ℕ, |)$ , the set of natural numbers ordered by divisibility, is not a well-quasi-order: the prime numbers are an infinite antichain (see Pic.2).
$(ℕ^{k}, \leq)$ , the set of vectors of $k$ natural numbers (where $k$ is finite) with component-wise ordering, is a well partial order (Dickson's lemma; see Pic.3). More generally, if $(X, \leq)$ is well-quasi-order, then $(X^{k}, \leq^{k})$ is also a well-quasi-order for all $k$ .
Let $X$ be an arbitrary finite set with at least two elements. The set $X^{*}$ of words over $X$ ordered lexicographically (as in a dictionary) is not a well-quasi-order because it contains the infinite decreasing sequence $b, a b, a a b, a a a b, \dots$ . Similarly, $X^{*}$ ordered by the prefix relation is not a well-quasi-order, because the previous sequence is an infinite antichain of this partial order. However, $X^{*}$ ordered by the subsequence relation is a well partial order. (If $X$ has only one element, these three partial orders are identical.)
More generally, $(X^{*}, \leq)$ , the set of finite $X$ -sequences ordered by embedding is a well-quasi-order if and only if $(X, \leq)$ is a well-quasi-order (Higman's lemma). Recall that one embeds a sequence $u$ into a sequence $v$ by finding a subsequence of $v$ that has the same length as $u$ and that dominates it term by term. When $(X, =)$ is an unordered set, $u \leq v$ if and only if $u$ is a subsequence of $v$ .
$(X^{ω}, \leq)$ , the set of infinite sequences over a well-quasi-order $(X, \leq)$ , ordered by embedding, is not a well-quasi-order in general. That is, Higman's lemma does not carry over to infinite sequences. Better-quasi-orderings have been introduced to generalize Higman's lemma to sequences of arbitrary lengths.
Embedding between finite trees with nodes labeled by elements of a wqo $(X, \leq)$ is a wqo (Kruskal's tree theorem).
Embedding between infinite trees with nodes labeled by elements of a wqo $(X, \leq)$ is a wqo (Nash-Williams' theorem).
Embedding between countable scattered linear order types is a well-quasi-order (Laver's theorem).
Embedding between countable boolean algebras is a well-quasi-order. This follows from Laver's theorem and a theorem of Ketonen.
Finite graphs ordered by a notion of embedding called "graph minor" is a well-quasi-order (Robertson–Seymour theorem).
Graphs of finite tree-depth ordered by the induced subgraph relation form a well-quasi-order, as do the cographs ordered by induced subgraphs.

Constructing new wpo's from given ones

Let $X_{1}$ and $X_{2}$ be two disjoint wpo sets. Let $Y = X_{1} \cup X_{2}$ , and define a partial order on $Y$ by letting $y_{1} \leq_{Y} y_{2}$ if and only if $y_{1}, y_{2} \in X_{i}$ for the same $i \in {1, 2}$ and $y_{1} \leq_{X_{i}} y_{2}$ . Then $Y$ is wpo, and $o (Y) = o (X_{1}) \oplus o (X_{2})$ , where $\oplus$ denotes natural sum of ordinals.

Given wpo sets $X_{1}$ and $X_{2}$ , define a partial order on the Cartesian product $Y = X_{1} \times X_{2}$ , by letting $(a_{1}, a_{2}) \leq_{Y} (b_{1}, b_{2})$ if and only if $a_{1} \leq_{X_{1}} b_{1}$ and $a_{2} \leq_{X_{2}} b_{2}$ . Then $Y$ is wpo (this is a generalization of Dickson's lemma), and $o (Y) = o (X_{1}) \otimes o (X_{2})$ , where $\otimes$ denotes natural product of ordinals.

Given a wpo set $X$ , let $X^{*}$ be the set of finite sequences of elements of $X$ , partially ordered by the subsequence relation. Meaning, let $(x_{1}, \dots, x_{n}) \leq_{X^{*}} (y_{1}, \dots, y_{m})$ if and only if there exist indices $1 \leq i_{1} < \dots < i_{n} \leq m$ such that $x_{j} \leq_{X} y_{i_{j}}$ for each $1 \leq j \leq n$ . By Higman's lemma, $X^{*}$ is wpo. The ordinal type of $X^{*}$ is $o (X^{*}) = {\begin{matrix} ω^{ω^{o (X) - 1}}, & o (X) finite; \\ ω^{ω^{o (X) + 1}}, & o (X) = ε_{α} + n for some α and some finite n; \\ ω^{ω^{o (X)}}, & otherwise . \end{matrix}$

Given a wpo set $X$ , let $T (X)$ be the set of all finite rooted trees whose vertices are labeled by elements of $X$ . Partially order $T (X)$ by the tree embedding relation. By Kruskal's tree theorem, $T (X)$ is wpo. This result is nontrivial even for the case $| X | = 1$ (which corresponds to unlabeled trees), in which case $o (T (X))$ equals the small Veblen ordinal. In general, for $o (X)$ countable, we have the upper bound $o (T (X)) \leq ϑ (Ω^{ω} o (X))$ in terms of the $ϑ$ ordinal collapsing function. (The small Veblen ordinal equals $ϑ (Ω^{ω})$ in this ordinal notation.)

Wqo's versus well partial orders

According to Milner 1985, no real gain in generality is obtained by considering quasi-orders rather than partial orders... it is simply more convenient to do so.

Observe that a wpo is a wqo, and that a wqo gives rise to a wpo between equivalence classes induced by the kernel of the wqo. For example, if we order $ℤ$ by divisibility, we end up with $n \equiv m$ if and only if $n = \pm m$ , so that $(ℤ, |) \approx (ℕ, |)$ .

Infinite increasing subsequences

If $(X, \leq)$ is wqo then every infinite sequence $x_{0}, x_{1}, x_{2}, \dots,$ contains an infinite increasing subsequence $x_{n_{0}} \leq x_{n_{1}} \leq x_{n_{2}} \leq \dots$ (with $n_{0} < n_{1} < n_{2} < \dots$ ). Such a subsequence is sometimes called perfect. This can be proved by a Ramsey argument: given some sequence $(x_{i})_{i}$ , consider the set $I$ of indexes $i$ such that $x_{i}$ has no larger or equal $x_{j}$ to its right, i.e., with $i < j$ . If $I$ is infinite, then the $I$ -extracted subsequence contradicts the assumption that $X$ is wqo. So $I$ is finite, and any $x_{n}$ with $n$ larger than any index in $I$ can be used as the starting point of an infinite increasing subsequence.

The existence of such infinite increasing subsequences is sometimes taken as a definition for well-quasi-ordering, leading to an equivalent notion.

Properties of wqos

Given a quasiordering $(X, \leq)$ the quasiordering $(P (X), \leq^{+})$ defined by $A \leq^{+} B ⟺ \forall a \in A, \exists b \in B, a \leq b$ is well-founded if and only if $(X, \leq)$ is a wqo.
A quasiordering is a wqo if and only if the corresponding partial order (obtained by quotienting by $x \sim y ⟺ x \leq y \land y \leq x$ ) has no infinite descending sequences or antichains. (This can be proved using a Ramsey argument as above.)
Given a well-quasi-ordering $(X, \leq)$ , any sequence of upward-closed subsets $S_{0} \subseteq S_{1} \subseteq \dots \subseteq X$ eventually stabilises (meaning there exists $n \in ℕ$ such that $S_{n} = S_{n + 1} = \dots$ ; a subset $S \subseteq X$ is called upward-closed if $\forall x, y \in X, x \leq y \land x \in S \Rightarrow y \in S$ ): assuming the contrary $\forall i \in ℕ, \exists j \in ℕ, j > i, \exists x \in S_{j} ∖ S_{i}$ , a contradiction is reached by extracting an infinite non-ascending subsequence.
Given a well-quasi-ordering $(X, \leq)$ , any subset $S$ of $X$ has a finite number of minimal elements with respect to $\leq$ , for otherwise the minimal elements of $S$ would constitute an infinite antichain.

Notes

↑ Here $x < y$ means: $x \leq y$ and not $y \leq x .$

References

↑ Towsner, Henry (2013). "Partial impredicativity in reverse mathematics". The Journal of Symbolic Logic. 78 (2): 459–488. doi:10.2178/jsl.7802070. JSTOR 43303662. MR 3145191. Page 471: "Q is a well-quasi-order iff the tree of bad sequences from Q is well-founded."
1 2 3 4 de Jongh, Dick H. G.; Parikh, Rohit (1977). "Well-partial orderings and hierarchies". Indagationes Mathematicae (Proceedings). 80 (3): 195–207. doi:10.1016/1385-7258(77)90067-1.
↑ Gasarch, W. (1998). "A survey of recursive combinatorics". Handbook of Recursive Mathematics, Vol. 2. Stud. Logic Found. Math. Vol. 139. Amsterdam: North-Holland. pp. 1041–1176. doi:10.1016/S0049-237X(98)80049-9. MR 1673598.. See in particular page 1160.
↑ Nešetřil, Jaroslav; Ossona de Mendez, Patrice (2012). "Lemma 6.13". Sparsity: Graphs, Structures, and Algorithms. Algorithms and Combinatorics. Vol. 28. Heidelberg: Springer. p. 137. doi:10.1007/978-3-642-27875-4. ISBN 978-3-642-27874-7. MR 2920058..
↑ Damaschke, Peter (1990). "Induced subgraphs and well-quasi-ordering". Journal of Graph Theory. 14 (4): 427–435. doi:10.1002/jgt.3190140406. MR 1067237..
↑ Schmidt, Diana (1979). Well-partial orderings and their maximal order types (Habilitationsschrift). Heidelberg. Republished in: Schmidt, Diana (2020). "Well-partial orderings and their maximal order types". In Schuster, Peter M.; Seisenberger, Monika; Weiermann, Andreas (eds.). Well-Quasi Orders in Computation, Logic, Language and Reasoning. Trends in Logic. Vol. 53. Springer. pp. 351–391. doi:10.1007/978-3-030-30229-0_13. ISBN 978-3-030-30228-3.
↑ Rathjen, Michael; Weiermann, Andreas (1993). "Proof-theoretic investigations on Kruskal's theorem". Annals of Pure and Applied Logic. 60: 49–88. doi:10.1016/0168-0072(93)90192-G.
↑ Milner, E. C. (1985). "Basic WQO- and BQO-theory". In Rival, I. (ed.). Graphs and Order. The Role of Graphs in the Theory of Ordered Sets and Its Applications. D. Reidel Publishing Co. pp. 487–502. ISBN 90-277-1943-8.
↑ Forster, Thomas (2003). "Better-quasi-orderings and coinduction". Theoretical Computer Science. 309 (1–3): 111–123. doi:10.1016/S0304-3975(03)00131-2.

v t e Order theory
Topics Glossary Category
Key concepts	Binary relation Boolean algebra Cyclic order Lattice Partially ordered set Preorder Total order Weak ordering
Results	Boolean prime ideal theorem Cantor–Bernstein theorem Cantor's isomorphism theorem Dilworth's theorem Dushnik–Miller theorem Hausdorff maximal principle Knaster–Tarski theorem Kruskal's tree theorem Laver's theorem Mirsky's theorem Szpilrajn extension theorem Zorn's lemma
Properties & Types (list)	Antisymmetric Asymmetric Boolean algebra topics Completeness Connected Covering Dense Directed (Partial) Equivalence Foundational Heyting algebra Homogeneous Idempotent Lattice Bounded Complemented Complete Distributive Join and meet Partially ordered set Chain-complete Eulerian Graded Locally finite Strict Prefix order Preorder Total Reflexive Semilattice Semiorder Symmetric Tolerance Total Transitive Well-founded Well-quasi-ordering (Better) (Pre) Well-order
Constructions	Composition Converse/Transpose Lexicographic order Linear extension Product order Reflexive closure Series-parallel partial order Star product Symmetric closure Transitive closure
Topology & Orders	Alexandrov topology & Specialization preorder Ordered topological vector space Normal cone Order topology Order topology Topological vector lattice Banach Fréchet Locally convex Normed
Related	Antichain Cofinal Cofinality Comparability Graph Duality Filter Hasse diagram Ideal Net Subnet Order morphism Embedding Isomorphism Order type Ordered field Positive cone of an ordered field Ordered vector space Partially ordered Positive cone of an ordered vector space Riesz space Partially ordered group Positive cone of a partially ordered group Upper set Young's lattice