# Ordered sum

2010 Mathematics Subject Classification: Primary: 06A [MSN][ZBL]

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 $x \in X$, $y \in Y$.

#### References

 [1] G. Birkhoff, "Lattice theory" , Colloq. Publ. , 25 , Amer. Math. Soc. (1973) [2] L.A. Skornyakov, "Elements of lattice theory" , Hindushtan Publ. Comp. (1977) (Translated from Russian)
How to Cite This Entry:
Ordered sum. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Ordered_sum&oldid=35421
This article was adapted from an original article by T.S. Fofanova (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article