are a key concept in algebraic topology, measuring the "holes" in a topological space. They're defined as quotient modules of cycles and boundaries, capturing essential structural information about the space.
Computing homology groups involves tools like free resolutions and long exact sequences. These techniques allow us to break down complex structures into simpler components, making it easier to calculate and understand the homology of various mathematical objects.
Homology Groups and Their Properties
Definition and Components of Homology Groups
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Homology group Hn(C∗) defined as the kerdn/im dn+1
kerdn consists of , elements of Cn that are mapped to zero by the boundary map dn
im dn+1 consists of , elements of Cn that are the image of the boundary map dn+1 from Cn+1
Quotient module kerdn/im dn+1 identifies n-cycles that differ by an n-boundary, capturing the "essential" n-dimensional holes in the complex
Invariants of Homology Groups
βn represent the rank of the free part of the n-th homology group Hn(C∗)
β0 counts the number of connected components
β1 counts the number of 1-dimensional holes or "loops"
β2 counts the number of 2-dimensional voids or "cavities"
of Hn(C∗) captures the non-free part of the homology group
Torsion elements have finite order, i.e., they become zero when multiplied by some non-zero scalar
Torsion can be represented by a list of the orders of the torsion subgroups (e.g., Z2,Z3)
Computational Tools for Homology
Free Resolutions
of a module M is an of free modules F∗ with a map ε:F0→M such that the sequence
⋯→F2→F1→F0εM→0
is exact
Free resolutions can be used to compute homology groups by applying the H∗ to the resolution
Hn(M)≅Hn(F∗) for all n≥0
Constructing a free resolution involves finding a generating set for the module and relations among the generators
Long Exact Sequences
is a tool for computing homology groups of a chain complex from the homology groups of simpler complexes
Given a short exact sequence of chain complexes
0→A∗→B∗→C∗→0
there is an associated long exact sequence in homology
⋯→Hn(A∗)→Hn(B∗)→Hn(C∗)∂Hn−1(A∗)→⋯
The ∂ relates the homology groups of the complexes A∗,B∗, and C∗
Long exact sequences can be used to compute unknown homology groups from known ones by exploiting the exactness property
Exactness means that the kernel of each map is equal to the image of the previous map