In topology, the degree of a continuous mapping between two compact oriented manifolds of the same dimension is a number that represents the number of times that the domain manifold wraps around the range manifold under the mapping. The degree is always an integer, but may be positive or negative depending on the orientations.
The degree of a map was first defined by Brouwer,[1] who showed that the degree is homotopy invariant (invariant among homotopies), and used it to prove the Brouwer fixed point theorem. In modern mathematics, the degree of a map plays an important role in topology and geometry. In physics, the degree of a continuous map (for instance a map from space to some order parameter set) is one example of a topological quantum number.
Sn
n=1
Let
f\colonSn\toSn
f
f*\colon
n\right) | |
H | |
n\left(S |
\to
n\right) | |
H | |
n\left(S |
Hn\left( ⋅ \right)
n
n\right)\congZ | |
H | |
n\left(S |
f*
f*\colonx\mapsto\alphax
\alpha\inZ
\alpha
f
Let X and Y be closed connected oriented m-dimensional manifolds. Poincare Duality implies that the manifold's top homology group is isomorphic to Z. Choosing an orientation means choosing a generator of the top homology group.
A continuous map f : X →Y induces a homomorphism f∗ from Hm(X) to Hm(Y). Let [''X''], resp. [''Y''] be the chosen generator of Hm(X), resp. Hm(Y) (or the fundamental class of X, Y). Then the degree of f is defined to be f*([''X'']). In other words,
f*([X])=\deg(f)[Y].
If y in Y and f −1(y) is a finite set, the degree of f can be computed by considering the m-th local homology groups of X at each point in f −1(y).Namely, if
f-1(y)=\{x1,...,xm\}
\deg(f)=
m | |
\sum | |
i=1 |
\deg(f| | |
xi |
).
In the language of differential topology, the degree of a smooth map can be defined as follows: If f is a smooth map whose domain is a compact manifold and p is a regular value of f, consider the finite set
f-1(p)=\{x1,x2,\ldots,xn\}.
By p being a regular value, in a neighborhood of each xi the map f is a local diffeomorphism. Diffeomorphisms can be either orientation preserving or orientation reversing. Let r be the number of points xi at which f is orientation preserving and s be the number at which f is orientation reversing. When the codomain of f is connected, the number r − s is independent of the choice of p (though n is not!) and one defines the degree of f to be r − s. This definition coincides with the algebraic topological definition above.
The same definition works for compact manifolds with boundary but then f should send the boundary of X to the boundary of Y.
One can also define degree modulo 2 (deg2(f)) the same way as before but taking the fundamental class in Z2 homology. In this case deg2(f) is an element of Z2 (the field with two elements), the manifolds need not be orientable and if n is the number of preimages of p as before then deg2(f) is n modulo 2.
Integration of differential forms gives a pairing between (C∞-)singular homology and de Rham cohomology: , where
c
c
\omega
\left\langlef*[c],[\omega]\right\rangle=\left\langle[c],f*[\omega]\right\rangle,
where f∗ and f∗ are induced maps on chains and forms respectively. Since f∗[''X''] = deg f · [''Y''], we have
\degf\intY\omega=\intXf*\omega
for any m-form ω on Y.
If
\Omega\subset\Rn
f:\bar\Omega\to\Rn
p
f
p\notinf(\partial\Omega)
\deg(f,\Omega,p)
\deg(f,\Omega,p):=
\sum | |
y\inf-1(p) |
sgn\det(Df(y))
Df(y)
f
y
This definition of the degree may be naturally extended for non-regular values
p
\deg(f,\Omega,p)=\deg\left(f,\Omega,p'\right)
p'
p
The degree satisfies the following properties:[2]
\deg\left(f,\bar\Omega,p\right) ≠ 0
x\in\Omega
f(x)=p
\deg(\operatorname{id},\Omega,y)=1
y\in\Omega
\Omega1,\Omega2
\Omega=\Omega1\cup\Omega2
y\not\inf{\left(\overline{\Omega}\setminus\left(\Omega1\cup\Omega2\right)\right)}
f
g
F(t)
F(0)=f,F(1)=g
p\notinF(t)(\partial\Omega)
\deg(f,\Omega,p)=\deg(g,\Omega,p)
p\mapsto\deg(f,\Omega,p)
\Rn-f(\partial\Omega)
These properties characterise the degree uniquely and the degree may be defined by them in an axiomatic way.
In a similar way, we could define the degree of a map between compact oriented manifolds with boundary.
The degree of a map is a homotopy invariant; moreover for continuous maps from the sphere to itself it is a complete homotopy invariant, i.e. two maps
f,g:Sn\toSn
\deg(f)=\deg(g)
In other words, degree is an isomorphism between
\left[Sn,Sn\right]=\pinSn
Z
Moreover, the Hopf theorem states that for any
n
f,g:M\toSn
\deg(f)=\deg(g).
A self-map
f:Sn\toSn
F:Bn+1\toSn
\deg(f)=0
Sn
There is an algorithm for calculating the topological degree deg(f, B, 0) of a continuous function f from an n-dimensional box B (a product of n intervals) to
\Rn