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A chain homotopy offers a way to relate two chain maps that induce the same map on homology groups, even though the maps may be different. Given two chain complexes A and B, and two chain maps f, g : A → B, a chain homotopy is a sequence of homomorphisms h n : A n → B n+1 such that hd A + d B h = f − g. The maps may be written out in a ...
Let = be a -graded algebra, with product , equipped with a map : of degree (homologically graded) or degree + (cohomologically graded). We say that (,,) is a differential graded algebra if is a differential, giving the structure of a chain complex or cochain complex (depending on the degree), and satisfies a graded Leibniz rule.
For example, a co-dimension 2 link in 3-dimensional space is a subspace of 3-dimensional Euclidean space (or often the 3-sphere) whose connected components are homeomorphic to circles. The simplest nontrivial example of a link with more than one component is called the Hopf link , which consists of two circles (or unknots ) linked together once.
Geometry lets us visualize what the inside of a knot or link complement looks like by imagining light rays as traveling along the geodesics of the geometry. An example is provided by the picture of the complement of the Borromean rings. The inhabitant of this link complement is viewing the space from near the red component.
Algebraic link diagram for the Borromean rings. The vertical dotted black midline is a Conway sphere separating the diagram into 2-tangles. In knot theory, the Borromean rings are a simple example of a Brunnian link, a link that cannot be separated but that falls apart into separate unknotted loops as soon as any one of its components is ...
The category of arbitrary chain-complexes of R-modules has a model structure that is defined by weak equivalences are chain homotopy equivalences of chain-complexes; cofibrations are monomorphisms that are split as morphisms of underlying R-modules; and; fibrations are epimorphisms that are split as morphisms of underlying R-modules.
Primeness and splittability of a link is easily seen from the diagram. The crossing number of a reduced, alternating diagram is the crossing number of the knot. This last is one of the celebrated Tait conjectures. An alternating knot diagram is in one-to-one correspondence with a planar graph. Each crossing is associated with an edge and half ...
Many hyperbolic lines through point P not intersecting line a in the Beltrami Klein model A hyperbolic triheptagonal tiling in a Beltrami–Klein model projection. In geometry, the Beltrami–Klein model, also called the projective model, Klein disk model, and the Cayley–Klein model, is a model of hyperbolic geometry in which points are represented by the points in the interior of the unit ...