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In mathematics, a homogeneous relation is called connected or total if it relates all distinct pairs of elements in one direction or the other, i.e., if any two elements can be compared. More formally, the homogeneous relation $R$ on a set $X$ is connected when for all $$x, y \in X,$$ where $$x \neq y$$,
 * $$x \mathrel{R} y \quad \text{or}\quad y \mathrel{R} x,$$

or, equivalently, when for all $$x, y \in X$$,
 * $$x \mathrel{R} y \quad \text{or}\quad y \mathrel{R} x \quad \text{or}\quad x = y.$$

A relation with the property that for all $$x, y \in X$$,
 * $$x \mathrel{R} y \quad \text{or}\quad y \mathrel{R} x$$

is called strongly connected.

Terminology for these properties is not uniform, see below. The notion should not be confused with that of a total relation in the sense that for all $$x \in X$$ there is a $$y \in X$$ so that $$x \mathrel{R} y$$ (see serial relation).

Connectedness and total orders
Connectedness features prominently in the definition of total orders: a total (or linear) order is a partial order in which any two elements are comparable, i.e., the order relation is connected. Similarly, a strict partial order that is connected is a strict total order.

A relation is a total order if, and only if, it is both a partial order and strongly connected. A relation is a strict total order if, and only if, it is a strict partial order and just connected. A strict total order can never be strongly connected (except on an empty domain).

Terminology
The main use of the notion of connected relation is in the context of orders, where it is used to define total, or linear, orders. In this context, the property is often not specifically named. Rather, total orders are defined as partial orders in which any two elements are comparable. Thus, total is used more generally for relations that are connected or strongly connected. However, this notion of "total relation" must be distinguished from the property of being serial, which is also called total. Similarly, connected relations are sometimes called complete, although this, too, can lead to confusion: The universal relation is also called complete, and "complete" has several other meanings in order theory. Connected relations are also called connex or.

When the relations considered are not orders, being connected and being strongly connected are importantly different properties. Sources which define both then use pairs of terms such as weakly connected and connected, complete and strongly complete, semiconnex and connex, or connex and strictly connex, respectively, as alternative names for the notions of connected and strongly connected as defined above.

Characterizations
Let $$R$$ be a homogeneous relation. The following are equivalent: where $$U$$ is the universal relation and $$R^\top$$ is the converse relation of $$R$$.
 * $$R$$ is strongly connected;
 * $$U \subseteq R \cup R^\top$$;
 * $$\overline{R} \subseteq R^\top$$;
 * $$\overline{R}$$ is asymmetric,

The following are equivalent: where $$\overline{I}$$ is the complementary relation of the identity relation $$I$$ and $$R^\top$$ is the converse relation of $$R$$.
 * $$R$$ is connected;
 * $$\overline{I} \subseteq R \cup R^\top$$;
 * $$\overline{R} \subseteq R^\top \cup I$$;
 * $$\overline{R}$$ is antisymmetric,

Properties

 * The relation $$E$$ of a tournament graph $$G$$ is always a connected relation on the set of $G$'s vertices.
 * If a strongly connected is symmetric, it is the universal relation.
 * A relation is strongly connected if, and only if, it is connected and reflexive.
 * A connected relation on a set $$X$$ cannot be antitransitive, provided $$X$$ has at least 4 elements. On a 3-element set $\{a, b, c\}$, e.g. the relation $\{(a, b), (b, c), (c, a)\}$ has both properties.
 * If $$R$$ is a connected relation on $$X$$, then all, or all but one, elements of $$X$$ are in the range of $$R$$. Similarly, all, or all but one, elements of $$X$$ are in the domain of $$R$$.