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A noncrossing partition of S is a partition in which no two blocks "cross" each other, i.e., if a and b belong to one block and x and y to another, they are not arranged in the order a x b y. If one draws an arch based at a and b , and another arch based at x and y , then the two arches cross each other if the order is a x b y but not if it is ...
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:
In mathematics, a covering number is the number of balls of a given size needed to completely cover a given space, with possible overlaps between the balls. The covering number quantifies the size of a set and can be applied to general metric spaces.
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 ...
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.
To define the join, form a relation on the blocks A of α and the blocks B of ρ by A ~ B if A and B are not disjoint. Then α ∨ ρ {\displaystyle \alpha \vee \rho } is the partition in which each block C is the union of a family of blocks connected by this relation.
This lemma is an intermediate step, of independent interest, in the proof of the Vitali covering theorem. The covering theorem is credited to the Italian mathematician Giuseppe Vitali. [1] The theorem states that it is possible to cover, up to a Lebesgue-negligible set, a given subset E of R d by a disjoint family extracted from a Vitali ...
The union of the tangent and secant lines (the secant variety) of a twisted cubic C fill up P 3 and the lines are pairwise disjoint, except at points of the curve itself. In fact, the union of the tangent and secant lines of any non-planar smooth algebraic curve is three-dimensional.