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A partition of a set S is a set of non-empty, pairwise disjoint subsets of S, called "parts" or "blocks", whose union is all of S.Consider a finite set that is linearly ordered, or (equivalently, for purposes of this definition) arranged in a cyclic order like the vertices of a regular n-gon.
However, the unit interval [0, 1] and the set of rational numbers Q are not almost disjoint, because their intersection is infinite. This definition extends to any collection of sets. A collection of sets is pairwise almost disjoint or mutually almost disjoint if any two distinct sets in the collection are almost disjoint. Often the prefix ...
Repeat step three until there is a new row with one more number than the previous row (do step 3 until = +) The number on the left hand side of a given row is the Bell number for that row. (,) Here are the first five rows of the triangle constructed by these rules:
The subdomains together must cover the whole domain; often it is also required that they are pairwise disjoint, i.e. form a partition of the domain. [5] In order for the overall function to be called "piecewise", the subdomains are usually required to be intervals (some may be degenerated intervals, i.e. single points or unbounded intervals).
By definition of wandering sets and since preserves , would thus contain a countably infinite union of pairwise disjoint sets that have the same -measure as . Since it was assumed μ ( X ) < ∞ {\displaystyle \mu (X)<\infty } , it follows that A {\displaystyle A} is a null set, and so all wandering sets must be null sets.
Then consider ,, …, to be a maximal collection of pairwise disjoint sets (that is, is the empty set unless =, and every set in intersects with some ). Because we assumed that W {\displaystyle W} had no sunflower of size r {\displaystyle r} , and a collection of pairwise disjoint sets is a sunflower, t < r {\displaystyle t<r} .
The vertex-connectivity statement of Menger's theorem is as follows: . Let G be a finite undirected graph and x and y two nonadjacent vertices. Then the size of the minimum vertex cut for x and y (the minimum number of vertices, distinct from x and y, whose removal disconnects x and y) is equal to the maximum number of pairwise internally disjoint paths from x to y.
In the case where P is ordered by inclusion, and closed under subsets, but does not contain the empty set, this is simply a family of pairwise disjoint sets. A strong upwards antichain B is a subset of P in which no two distinct elements have a common upper bound in P. Authors will often omit the "upwards" and "downwards" term and merely refer ...