Joint (mathématiques)

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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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.

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