Résumé
In topology, a field of mathematics, the join of two topological spaces and , often denoted by or , is a topological space formed by taking the disjoint union of the two spaces, and attaching line segments joining every point in to every point in . The join of a space with itself is denoted by . The join is defined in slightly different ways in different contexts If and are subsets of the Euclidean space , then:,that is, the set of all line-segments between a point in and a point in . Some authors restrict the definition to subsets that are joinable: any two different line-segments, connecting a point of A to a point of B, meet in at most a common endpoint (that is, they do not intersect in their interior). Every two subsets can be made "joinable". For example, if is in and is in , then and are joinable in . The figure above shows an example for m=n=1, where and are line-segments. The join of two simplices is a simplex: the join of an n-dimensional and an m-dimensional simplex is an (m+n+1)-dimensional simplex. Some special cases are: The join of two disjoint points is an interval (m=n=0). The join of a point and an interval is a triangle (m=0, n=1). The join of two line segments is homeomorphic to a solid tetrahedron or disphenoid, illustrated in the figure above right (m=n=1). The join of a point and an (n-1)-dimensional simplex is an n-dimensional simplex. The join of a point and a polygon (or any polytope) is a pyramid, like the join of a point and square is a square pyramid. The join of a point and a cube is a cubic pyramid. The join of a point and a circle is a cone, and the join of a point and a sphere is a hypercone. If and are any topological spaces, then: where the cylinder is attached to the original spaces and along the natural projections of the faces of the cylinder: Usually it is implicitly assumed that and are non-empty, in which case the definition is often phrased a bit differently: instead of attaching the faces of the cylinder to the spaces and , these faces are simply collapsed in a way suggested by the attachment projections : we form the quotient space where the equivalence relation is generated by At the endpoints, this collapses to and to .
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Publications associées (1)

The loop group and the cobar construction

Kathryn Hess Bellwald

We prove that for any 1-reduced simplicial set X, Adams' cobar construction, on the normalised chain complex of X is naturally a strong deformation retract of the normalised chains CGX on the Kan loop group GX, opening up the possibility of applying the to ...
2010
Concepts associés (2)
Cône (topologie)
En topologie, et en particulier en topologie algébrique, le cône CX d'un espace topologique X est l'espace quotient :du produit de X par l'intervalle unité I = [0, 1]. Intuitivement, on forme un cylindre de base X et on réduit une extrémité du cylindre à un point. Le cône construit sur un point p de la droite réelle est le segment {p} × [0,1]. Le cône construit sur deux points {0,1} est un "V" avec les extrémités en 0 et 1. Le cône construit sur un intervalle I de la droite réelle est un triangle plein, aussi appelé 2-simplexe (voir l'exemple final).
Espace contractile
En mathématiques, un espace topologique est dit contractile s'il est homotopiquement équivalent à un point. Tous ses groupes d'homotopie sont donc triviaux, ainsi que ses groupes d'homologie de degré > 0. Tout espace vectoriel normé (ou même : tout espace vectoriel topologique sur R) est contractile, à commencer par la droite réelle et le plan complexe. Plus généralement, toute partie étoilée d'un tel espace (en particulier : tout convexe non vide, comme un intervalle réel ou un disque) est clairement contractile.