In algebraic topology, the Betti numbers are used to distinguish topological spaces based on the connectivity of n-dimensional simplicial complexes. For the most reasonable finite-dimensional spaces (such as compact manifolds, finite simplicial complexes or CW complexes), the sequence of Betti numbers is 0 from some point onward (Betti numbers vanish above the dimension of a space), and they are all finite.

The n^th Betti number represents the rank of the n^th homology group, denoted H_n, which tells us the maximum number of cuts that can be made before separating a surface into two pieces or 0-cycles, 1-cycles, etc.^[1] For example, if ${\displaystyle H_{n}(X)\cong 0}$ then ${\displaystyle b_{n}(X)=0}$, if ${\displaystyle H_{n}(X)\cong \mathbb {Z} }$ then ${\displaystyle b_{n}(X)=1}$, if ${\displaystyle H_{n}(X)\cong \mathbb {Z} \oplus \mathbb {Z} }$ then ${\displaystyle b_{n}(X)=2}$, if ${\displaystyle H_{n}(X)\cong \mathbb {Z} \oplus \mathbb {Z} \oplus \mathbb {Z} }$ then ${\displaystyle b_{n}(X)=3}$, etc. Note that only the ranks of infinite groups are considered, so for example if ${\displaystyle H_{n}(X)\cong \mathbb {Z} ^{k}\oplus \mathbb {Z} /(2)}$, where ${\displaystyle \mathbb {Z} /(2)}$ is the finite cyclic group of order 2, then ${\displaystyle b_{n}(X)=k}$. These finite components of the homology groups are their torsion subgroups, and they are denoted by torsion coefficients.

The term "Betti numbers" was coined by Henri Poincaré after Enrico Betti. The modern formulation is due to Emmy Noether. Betti numbers are used today in fields such as simplicial homology, computer science and digital images.

Geometric interpretation

Informally, the kth Betti number refers to the number of k-dimensional holes on a topological surface. A "k-dimensional hole" is a k-dimensional cycle that is not a boundary of a (k+1)-dimensional object.

The first few Betti numbers have the following definitions for 0-dimensional, 1-dimensional, and 2-dimensional simplicial complexes:

• b₀ is the number of connected components;
• b₁ is the number of one-dimensional or "circular" holes;
• b₂ is the number of two-dimensional "voids" or "cavities".

Thus, for example, a torus has one connected surface component so b₀ = 1, two "circular" holes (one equatorial and one meridional) so b₁ = 2, and a single cavity enclosed within the surface so b₂ = 1.

Another interpretation of b_k is the maximum number of k-dimensional curves that can be removed while the object remains connected. For example, the torus remains connected after removing two 1-dimensional curves (equatorial and meridional) so b₁ = 2.^[2]

The two-dimensional Betti numbers are easier to understand because we can see the world in 0, 1, 2, and 3-dimensions.

Formal definition

For a non-negative integer k, the kth Betti number b_k(X) of the space X is defined as the rank (number of linearly independent generators) of the abelian group H_k(X), the kth homology group of X. The kth homology group is ${\displaystyle H_{k}=\ker \delta _{k}/\operatorname {Im} \delta _{k+1}}$, the ${\displaystyle \delta _{k}}$s are the boundary maps of the simplicial complex and the rank of H_k is the kth Betti number. Equivalently, one can define it as the vector space dimension of H_k(X; Q) since the homology group in this case is a vector space over Q. The universal coefficient theorem, in a very simple torsion-free case, shows that these definitions are the same.

More generally, given a field F one can define b_k(X, F), the kth Betti number with coefficients in F, as the vector space dimension of H_k(X, F).

Poincaré polynomial

The Poincaré polynomial of a surface is defined to be the generating function of its Betti numbers. For example, the Betti numbers of the torus are 1, 2, and 1; thus its Poincaré polynomial is ${\displaystyle 1+2x+x^{2}}$. The same definition applies to any topological space which has a finitely generated homology.

Given a topological space which has a finitely generated homology, the Poincaré polynomial is defined as the generating function of its Betti numbers, via the polynomial where the coefficient of ${\displaystyle x^{n}}$ is ${\displaystyle b_{n}}$.

Examples

Betti numbers of a graph

Consider a topological graph G in which the set of vertices is V, the set of edges is E, and the set of connected components is C. As explained in the page on graph homology, its homology groups are given by:

${\displaystyle H_{k}(G)={\begin{cases}\mathbb {Z} ^{|C|}&k=0\\\mathbb {Z} ^{|E|+|C|-|V|}&k=1\\\{0\}&{\text{otherwise}}\end{cases}}}$

This may be proved straightforwardly by mathematical induction on the number of edges. A new edge either increments the number of 1-cycles or decrements the number of connected components.

Therefore, the "zero-th" Betti number b₀(G) equals |C|, which is simply the number of connected components.^[3]

The first Betti number b₁(G) equals |E| + |C| - |V|. It is also called the cyclomatic number—a term introduced by Gustav Kirchhoff before Betti's paper.^[4] See cyclomatic complexity for an application to software engineering.

All other Betti numbers are 0.

Betti numbers of a simplicial complex

Consider a simplicial complex with 0-simplices: a, b, c, and d, 1-simplices: E, F, G, H and I, and the only 2-simplex is J, which is the shaded region in the figure. There is one connected component in this figure (b₀); one hole, which is the unshaded region (b₁); and no "voids" or "cavities" (b₂).

This means that the rank of ${\displaystyle H_{0}}$ is 1, the rank of ${\displaystyle H_{1}}$ is 1 and the rank of ${\displaystyle H_{2}}$ is 0.

The Betti number sequence for this figure is 1, 1, 0, 0, ...; the Poincaré polynomial is ${\displaystyle 1+x\,}$.

Betti numbers of the projective plane

The homology groups of the projective plane P are:^[5]

${\displaystyle H_{k}(P)={\begin{cases}\mathbb {Z} &k=0\\\mathbb {Z} _{2}&k=1\\\{0\}&{\text{otherwise}}\end{cases}}}$

Here, Z₂ is the cyclic group of order 2. The 0-th Betti number is again 1. However, the 1-st Betti number is 0. This is because H₁(P) is a finite group - it does not have any infinite component. The finite component of the group is called the torsion coefficient of P. The (rational) Betti numbers b_k(X) do not take into account any torsion in the homology groups, but they are very useful basic topological invariants. In the most intuitive terms, they allow one to count the number of holes of different dimensions.

Properties

Euler characteristic

For a finite CW-complex K we have

${\displaystyle \chi (K)=\sum _{i=0}^{\infty }(-1)^{i}b_{i}(K,F),\,}$

where ${\displaystyle \chi (K)}$ denotes Euler characteristic of K and any field F.

Zdroj:https://en.wikipedia.org?pojem=Betti_number
Text je dostupný za podmienok Creative Commons Attribution/Share-Alike License 3.0 Unported; prípadne za ďalších podmienok. Podrobnejšie informácie nájdete na stránke Podmienky použitia.

---