Definition:Semiring of Sets

From ProofWiki
Jump to navigation Jump to search

Definition

A semiring of sets or semi-ring of sets is a system of sets $\SS$, subject to:

$(1):\quad \O \in \SS$
$(2):\quad A, B \in \SS \implies A \cap B \in \SS$; that is, $\SS$ is $\cap$-stable
$(3):\quad$ If $A, A_1 \in \SS$ such that $A_1 \subseteq A$, then there exists a finite sequence $A_2, A_3, \ldots, A_n \in \SS$ such that:
$(3a):\quad \displaystyle A = \bigcup_{k \mathop = 1}^n A_k$
$(3b):\quad$ The $A_k$ are pairwise disjoint

Alternatively, criterion $(3)$ can be replaced by:

$(3'):\quad$ If $A, B \in \SS$, then there exists a finite sequence of pairwise disjoint sets $A_1, A_2, \ldots, A_n \in \SS$ such that $\displaystyle A \setminus B = \bigcup_{k \mathop = 1}^n A_k$.


Some sources specify that a semiring of sets has to be non-empty, but as one of the conditions is that $\O \in \SS$, this criterion is superfluous.


Also see

  • Results about semirings of sets can be found here.


Sources