Ordered sum

2020 Mathematics Subject Classification: Primary: 06A [MSN][ZBL]

of partially ordered sets

An operation which associates with a system of disjoint partially ordered sets $\{P_\alpha : \alpha \in L \}$, where the index set $L$ is also partially ordered, a new partially ordered set $$ P = \coprod_{\alpha \in L} P_\alpha $$ the underlying set being the disjoint union of the sets $\{P_\alpha : \alpha \in L \}$, with order defined as follows. On the set $P$ one has $a \le b$ if and only if either $a,b \in P_\alpha$ and $a \le b$ in $P_\alpha$, for some $\alpha$, or $a \in P_\alpha$, $b \in P_\beta$ and $\alpha < \beta$ in $L$. Important particular cases of ordered sums are the cardinal and ordinal sums. The first of these is obtained when $L$ is a trivially ordered set, i.e. each of its elements is comparable only to itself, and the second when $L$ is a totally ordered set. Thus, in the cardinal sum of two disjoint partially ordered sets $X$ and $Y$ the relation $x \le y$ retains its meaning in the components $X$ and $Y$, while $x \in X$ and $y \in Y$ are incomparable; in the ordinal sum of $X$ and $Y$ the order relation is again preserved in the components and $x < y$ for all $x \in X$, $y \in Y$.

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