Join



Geometric join of two line segments. The original spaces are shown in green and blue. The join is a three-dimensional solid in gray.

In topology, a field of mathematics, the join of two topological spacesA and B, often denoted by AB{displaystyle Aast B} or AB{displaystyle Astar B}, is defined to be the quotient space

Before you join the meeting, you’ll see a preview screen with options to activate your Mic and Camera. When the session starts, you can access Attendees, Chat, and Mic & Camera settings using the menu bar in the top-right corner. 59 synonyms of join from the Merriam-Webster Thesaurus, plus 80 related words, definitions, and antonyms. Find another word for join. Join: to be adjacent to. Synonyms: abut, adjoin, border (on) Antonyms: demit, break up, dissever Find the right word. GAMES BROWSE THESAURUS WORD OF THE DAY WORDS AT PLAY.

Your teacher will give you a CODE to join the lesson. Join tables based on an inequality of field values. Joins do not have to be based on the equivalence of the joined fields. A join can be based on any comparison operator, such as greater than , less than. Joins that are not based on equivalence are called unequal joins. Join We understand the unique bond you share with other veterans of overseas conflicts because we, too, have been there. Since 1899, the VFW has been the nation’s leading veterans service organization in the fight to better the lives of all those who’ve worn the uniform of the United States military.

(A×B×I)/R,{displaystyle (Atimes Btimes I)/R,}

where I is the interval [0, 1] and R is the equivalence relation generated by

(a,b1,0)(a,b2,0)for all aA and b1,b2B,{displaystyle (a,b_{1},0)sim (a,b_{2},0)quad {mbox{for all }}ain A{mbox{ and }}b_{1},b_{2}in B,}
(a1,b,1)(a2,b,1)for all a1,a2A and bB.{displaystyle (a_{1},b,1)sim (a_{2},b,1)quad {mbox{for all }}a_{1},a_{2}in A{mbox{ and }}bin B.}

At the endpoints, this collapses A×B×{0}{displaystyle Atimes Btimes {0}} to A{displaystyle A} and A×B×{1}{displaystyle Atimes Btimes {1}} to B{displaystyle B}.

Intuitively, AB{displaystyle Astar B} is formed by taking the disjoint union of the two spaces and attaching line segments joining every point in A to every point in B.

Examples[edit]

  • The join of a space X with a one-point space is called the coneCX of X.
  • The join of a space X with S0{displaystyle S^{0}} (the 0-dimensional sphere, or, the discrete space with two points) is called the suspensionSX{displaystyle SX} of X.
  • The join of the spheres Sn{displaystyle S^{n}} and Sm{displaystyle S^{m}} is the sphere Sn+m+1{displaystyle S^{n+m+1}}.
  • The join of two pairs of isolated points is a square (without interior). The join of a square with a third pair of isolated points is an octahedron (again, without interior). In general, the join of n+1 pairs of isolated points is an n-dimensional octahedral sphere.
  • The join of two abstract simplicial complexes X and Y on disjoint vertex sets is the abstract simplicial complex {xy|xX,yY}{displaystyle {xcup y|xin X,yin Y}}. I.e., any simplex in the join is the union of a simplex from X and a simplex from Y. For example, if each of X and Y contain two isolated points, X = { {1}, {2} } and Y = { {3}, {4} }, then X * Y = { {1,3} , {1,4} , {2,3} , {2,4} } = a 'square' graph.

Properties[edit]

  • The join of two spaces is homeomorphic to a sum of cartesian products of cones over the spaces and the spaces themselves, where the sum is taken over the cartesian product of the spaces:
ABC(A)×BA×BC(B)×A.{displaystyle Astar Bcong C(A)times Bcup _{Atimes B}C(B)times A.}
  • Given basepointed CW complexes (A,a0) and (B,b0), the 'reduced join'
ABA{b0}{a0}B{displaystyle {frac {Astar B}{Astar {b_{0}}cup {a_{0}}star B}}}

is homeomorphic to the reduced suspension

Σ(AB){displaystyle Sigma (Awedge B)}

of the smash product. Consequently, since A{b0}{a0}B{displaystyle {Astar {b_{0}}cup {a_{0}}star B}} is contractible, there is a homotopy equivalence

ABΣ(AB).{displaystyle Astar Bsimeq Sigma (Awedge B).}

See also[edit]

References[edit]

  • Hatcher, Allen, Algebraic topology. Cambridge University Press, Cambridge, 2002. xii+544 pp. ISBN0-521-79160-X and ISBN0-521-79540-0
  • This article incorporates material from Join on PlanetMath, which is licensed under the Creative Commons Attribution/Share-Alike License.
  • Brown, Ronald, Topology and Groupoids Section 5.7 Joins.
Retrieved from 'https://en.wikipedia.org/w/index.php?title=Join_(topology)&oldid=994874237'

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