Eisenbud–Levine–Khimshiashvili signature formula

Template:Short description In mathematics, and especially differential topology and singularity theory, the Eisenbud–Levine–Khimshiashvili signature formula gives a way of computing the Poincaré–Hopf index of a real, analytic vector field at an algebraically isolated singularity. It is named after David Eisenbud, Harold I. Levine, and George Khimshiashvili. Intuitively, the index of a vector field near a zero is the number of times the vector field wraps around the sphere. Because analytic vector fields have a rich algebraic structure, the techniques of commutative algebra can be brought to bear to compute their index. The signature formula expresses the index of an analytic vector field in terms of the signature of a certain quadratic form.

Nomenclature

Consider the n-dimensional space Rⁿ. Assume that Rⁿ has some fixed coordinate system, and write x for a point in Rⁿ, where Template:Nowrap

Let X be a vector field on Rⁿ. For Template:Nowrap there exist functions Template:Nowrap such that one may express X as

$X=f_1(𝐱)\frac{\partial }{\partial x_1}+\cdots +f_n(𝐱)\frac{\partial }{\partial x_n}.$

To say that X is an analytic vector field means that each of the functions Template:Nowrap is an analytic function. One says that X is singular at a point p in Rⁿ (or that p is a singular point of X) if Template:Nowrap, i.e. X vanishes at p. In terms of the functions Template:Nowrap it means that Template:Nowrap for all Template:Nowrap. A singular point p of X is called isolated (or that p is an isolated singularity of X) if Template:Nowrap and there exists an open neighbourhood Template:Nowrap, containing p, such that Template:Nowrap for all q in U, different from p. An isolated singularity of X is called algebraically isolated if, when considered over the complex domain, it remains isolated.

Since the Poincaré–Hopf index at a point is a purely local invariant (cf. Poincaré–Hopf theorem), one may restrict one's study to that of germs. Assume that each of the ƒ_k from above are function germs, i.e. Template:Nowrap In turn, one may call X a vector field germ.

Construction

Let A_n,0 denote the ring of analytic function germs Template:Nowrap. Assume that X is a vector field germ of the form

$X=f_1(𝐱)\frac{\partial }{\partial x_1}+\cdots +f_n(𝐱)\frac{\partial }{\partial x_n}$

with an algebraically isolated singularity at 0. Where, as mentioned above, each of the ƒ_k are function germs Template:Nowrap. Denote by I_X the ideal generated by the ƒ_k, i.e. Template:Nowrap Then one considers the local algebra, B_X, given by the quotient

$B_X:=A_{n,0}/I_X.$

The Eisenbud–Levine–Khimshiashvili signature formula states that the index of the vector field X at 0 is given by the signature of a certain non-degenerate bilinear form (to be defined below) on the local algebra B_X.

The dimension of $B_X$ is finite if and only if the complexification of X has an isolated singularity at 0 in Cⁿ; i.e. X has an algebraically isolated singularity at 0 in Rⁿ. In this case, B_X will be a finite-dimensional, real algebra.

Definition of the bilinear form

Using the analytic components of X, one defines another analytic germ Template:Nowrap given by

$F(𝐱):=(f_1(𝐱),\dots ,f_n(𝐱)),$

for all Template:Nowrap. Let Template:Nowrap denote the determinant of the Jacobian matrix of F with respect to the basis Template:Nowrap Finally, let Template:Nowrap denote the equivalence class of J_F, modulo I_X. Using ∗ to denote multiplication in B_X one is able to define a non-degenerate bilinear form β as follows:

$\beta :B_X\times B_X\stackrel{*}{⟶}{B}_X\stackrel{\ell }{⟶}ℝ;\beta (g,h)=\ell (g*h),$

where $\ell$ is any linear function such that

$\ell \left(\left[J_F\right]\right)>0.$

As mentioned: the signature of β is exactly the index of X at 0.

Example

Consider the case Template:Nowrap of a vector field on the plane. Consider the case where X is given by

$X:=(x^3-3x{y}^2)\frac{\partial }{\partial x}+(3x^{2}y-y^3)\frac{\partial }{\partial y}.$

Clearly X has an algebraically isolated singularity at 0 since Template:Nowrap if and only if Template:Nowrap The ideal I_X is given by Template:Nowrap and

$B_X=A_{2,0}/(x^3-3x{y}^2,3x^{2}y-y^3)\cong ℝ⟨1,x,y,x^2,xy,y^2,x{y}^2,y^3,y^4⟩.$

The first step for finding the non-degenerate, bilinear form β is to calculate the multiplication table of B_X; reducing each entry modulo I_X. Whence

∗	1	x	y	x2	xy	y2	xy2	y3	y4
1	1	x	y	x2	xy	y2	xy2	y3	y4
x	x	x2	xy	3xy3	y3/3	xy2	y4/3	0	0
y	y	xy	y2	y3/3	xy2	y3	0	y4	0
x2	x2	3xy2	y3/3	y4	0	y4/3	0	0	0
xy	xy	y3/3	xy2	0	y4/3	0	0	0	0
y2	y2	xy2	y3	y4/3	0	y4	0	0	0
xy2	xy2	y4/3	0	0	0	0	0	0	0
y3	y3	0	y4	0	0	0	0	0	0
y4	y4	0	0	0	0	0	0	0	0

Direct calculation shows that Template:Nowrap, and so Template:Nowrap Next one assigns values for $\ell$. One may take

$\ell (1)=\ell (x)=\ell (y)=\ell (x^2)=\ell (xy)=\ell (y^2)=\ell (x{y}^2)=\ell (y^3)=0,\text{and}\ell (y^4)=3.$

This choice was made so that $\ell \left(\left[J_F\right]\right)>0$ as was required by the hypothesis, and to make the calculations involve integers, as opposed to fractions. Applying this to the multiplication table gives the matrix representation of the bilinear form β with respect to the given basis: $$\left[\begin{array}{ccccccccc}0& 0& 0& 0& 0& 0& 0& 0& 3\\ 0& 0& 0& 0& 0& 0& 1& 0& 0\\ 0& 0& 0& 0& 0& 0& 0& 3& 0\\ 0& 0& 0& 3& 0& 1& 0& 0& 0\\ 0& 0& 0& 0& 1& 0& 0& 0& 0\\ 0& 0& 0& 1& 0& 3& 0& 0& 0\\ 0& 1& 0& 0& 0& 0& 0& 0& 0\\ 0& 0& 3& 0& 0& 0& 0& 0& 0\\ 3& 0& 0& 0& 0& 0& 0& 0& 0\end{array}\right]$$ The eigenvalues of this matrix are Template:Nowrap There are 3 negative eigenvalues (Template:Nowrap), and six positive eigenvalues (Template:Nowrap); meaning that the signature of β is Template:Nowrap. It follows that X has Poincaré–Hopf index +3 at the origin.

Topological verification

With this particular choice of X it is possible to verify the Poincaré–Hopf index is +3 by a direct application of the definition of Poincaré–Hopf index. This is very rarely the case, and was the reason for the choice of example. If one takes polar coordinates on the plane, i.e. Template:Nowrap and Template:Nowrap then Template:Nowrap and Template:Nowrap Restrict X to a circle, centre 0, radius Template:Nowrap, denoted by C_0,ε; and consider the map Template:Nowrap given by

$G:X⟼\frac{X}{||X||}.$

The Poincaré–Hopf index of X is, by definition, the topological degree of the map G. Restricting X to the circle C_0,ε, for arbitrarily small ε, gives

$G(\theta )=(\cos (3\theta ),\sin (3\theta )),$

meaning that as θ makes one rotation about the circle C_0,ε in an anti-clockwise direction; the image G(θ) makes three complete, anti-clockwise rotations about the unit circle C_0,1. Meaning that the topological degree of G is +3 and that the Poincaré–Hopf index of X at 0 is +3.