M721: Index Theory

Ruminations of a Graduate Class

The Dirac operator

with one comment

We begin by endowing a vector bundle with a Clifford module structure. It is with additional structure that we may define a Dirac operator.

Let (X^n, g) be an n-dimensional Riemannian manifold with covariant derivative \nabla^X (on TX) , and let \mathbb{R}^N\to S\to X be a vector bundle.

Clifford Module Bundles and a Dirac “Type” Operator

Definition (Clifford module)

A Clifford module T for a real inner product space (V,\beta) is a left module over Cl(V,\beta). Equivalently, there is a -algebra homomorphism c: Cl(V,\beta) \to \text{Hom(T,T)} given by c(v) = (t\mapsto v\cdot t). Since v^2:=v\cdot v = -||v||^2 for any v\in Cl(V,\beta) (see the Glossary below), one has that c satisfies c(v)^2 = - ||v||^2\text{Id}.

Definition (Bundle of Clifford Modules)

A bundle S (as above) is a bundle of Clifford modules if there is a map of bundles of \mathbb{R}-algebras C:Cl(TX)\to \text{Hom}(S,S) such that c(v)^2s = -||v||^2 s for any section s\in C^{\infty}(S). In other words, for each x\in X, \text{Hom}(S_x, S_x) is a Clifford module for (T_x X, g_x).

Definition (Dirac type operator)

Let S be a Clifford module bundle equipped with a covariant derivative \nabla^S. Let D:C^\infty(S)\to C^\infty(S) be the map defined by the composition

C^\infty(S)=\Omega^0(S) \xrightarrow{\nabla^S} \Omega^1(S) = C^\infty(\text{Hom}(TX,S)) \xrightarrow{H} C^\infty(TX\otimes S)\xrightarrow{c} \Omega^0(S)

where H is the inverse of the bundle isomorphism C^\infty(TX\otimes S) \to C^\infty(\text{Hom}(TX,S)), v\otimes s \mapsto g(v,.)\otimes s, and where c(v\otimes s) = v\cdot s is Clifford multiplication. We call such a map (which depends on the Clifford module bundle S, \nabla^S and g) a Dirac type operator.

If we a fix an orthonormal frame \{e_1,\ldots, e_n\} for TX over some neighborhood U\subset X and, using the metric g, let \{e^i\} be the corresponding frame for T^\ast X|_U, we may write this composition locally as

s\overset{\nabla^S}{\mapsto} \sum_{i=1}^n e^i\otimes \nabla^S_{e_i} s \overset{H}{\mapsto} \sum_{i=1}^n e_i\otimes \nabla^S_{e_i} s \overset{c}{\mapsto} \sum_{i=1}^n e_i\cdot \nabla^S_{e_i} s

That is, a Dirac type operator is locally of the form

Ds = \sum_{i=1}^n e_i\cdot \nabla^S_{e_i} s.

Proposition

A Dirac type operator is a first order differential operator.

Proof

Let s\in C^\infty(S) and f\in C^{\infty}(X). Using the local description above, we compute:

[D, f]s = D(fs) - f(Ds)

= \sum_{i=1}^n \bigl[ e_i\cdot \nabla^S_{e_i} (fs) - f e_i\cdot \nabla^S_{e_i} s\bigr]

= \sum_{i=1}^n \bigl[ e_i\cdot (e_i(f)s + f \nabla^S_{e_i} s) - f  e_i\cdot \nabla^S_{e_i} s\bigr]

= \sum_{i=1}^n \bigl[ e_i\cdot e_i(f)s +  e_i\cdot f \nabla^S_{e_i} s - f  e_i\cdot \nabla^S_{e_i} s \bigr]

= \sum_{i=1}^n \bigl[ e_i\cdot e_i(f)s + f e_i\cdot \nabla^S_{e_i} s - f  e_i\cdot \nabla^S_{e_i} s \bigr]

= \sum_{i=1}^n e_i\cdot e_i(f)s.

In particular, [D,f](gs) = g[D,f](s) for g\in C^{\infty}(X), so [D,f] is C^\infty(X)-linear and hence in DO_0(S,S). Thus D\in DO_1(S,S), and as D itself is not C^\infty(X)-linear, D is of order 1. \square

Remark

The above proof extends (by incorporating induction) to show that the composition of a k– and an m- order differential operator is a differential operator of order \leq k+m. Here, \nabla^S and c are differential operators of order 1 and 0, respectively.

We next show that D is elliptic.

Lemma (Symbol of a Dirac type operator)

Let D be a Dirac type operator and let \xi^\ast\in T^\ast X. Then the symbol \sigma_D(\xi^\ast) of D at \xi^\ast is given by

\sigma_D(\xi^\ast)(s) = \frac{1}{i} \xi\cdot s,

where \xi\in TX is the dual to \xi^\ast determined by the metric, i.e., such that \xi^\ast = g(\xi, \cdot).

Proof

Fix x\in X, and let U be an open neighborhood of x in X. Using g choose an orthonormal frame \{e_i\} of TX|_U with dual frame \{e^i\} for T^\ast X|_U. Being a bundle homomorphism \pi^\ast(S)\to \pi^\ast(S) (over \mathbb{R}), \sigma_D is \mathbb{R}-linear in the T^\ast X-ordinate. Thus it suffices to verify the proposition for e^j; that is, we wish to show that \sigma_D(e^j)(s) = \frac{1}{i} e_j\cdot s for s\in C^{\infty}(S|_U).

Choose a local chart \varphi: U\to \mathbb{R}^n such that \varphi(x)=0; let x_i=\text{pr}_i\circ \varphi: U\to \mathbb{R}. Note that \{e_i(x)\} is an orthonormal basis for T_xX. Let \frac{\partial}{\partial x_i} = d\varphi_x(e_i(x)); then {dx_j}_x(e_i(x)) = {d\text{pr}_j}|_0\circ d\varphi_x(e_i(x)) =  {d\text{pr}_j}|_0\bigl(\frac{\partial}{\partial x_i}\bigr) = \delta_{ji}, so {dx_j}_x = e^j(x).

Thus (see the Glossary below), as D is order 1, we have

\sigma_D(e^j)(s(x)) = \frac{1}{i} D(x_j s)(x)

= \frac{1}{i} (\sum_{i=1}^n e_i\cdot \nabla_{e_i}(x_j s))(x)

= \frac{1}{i} \sum_{i=1}^n e_i(x) \cdot \bigl[ {dx_j}_x(e_i(x)) s(x) + \underbrace{x_j(x)}_{=0}\nabla_{e_i}s(x)\bigr]

= \frac{1}{i} \sum_{i=1}^n e_i(x) \cdot \bigl[ \delta_{ji} s(x) + 0]

= \frac{1}{i} e_j(x)\cdot s(x),

as required. \square

Corollary

A Dirac type operator is elliptic.

Proof

If \xi^\ast\in TX^\ast-X then \sigma_D(\xi^\ast) has inverse \sigma_D^{-1}(\xi^\ast)(s) = i \frac{1}{\xi^\ast} \cdot s . \square

Remark

Noting that \sigma_{D^2}(\xi) = \sigma_D(\xi)\sigma_D(\xi) (the product of linear maps), we observe that \sigma_{D^2}(\xi)s = -\xi\cdot \xi \cdot s = ||\xi||^2 s. Taking S=Cl(TX), this — apparently — implies that a Dirac type operator is, at the symbol level, the square root of the Laplacian.

\mathbb{Z}_2-grading and “a” Dirac Operator

A Dirac type operator is formally self-adjoint (so that its index is 0) if we impose the following further restrictions on the Clifford-module bundle.

Definition (Clifford-Compatible)

Let S\to X be a bundle of Clifford modules. We say that S is Clifford compatible if it is equipped with a metric \langle, \rangle and a covariant derivative \nabla^S such that

(1) \nabla^S is Riemannian, i.e., for all sections s, t\in C^\infty(S):

d\langle s, t\rangle = \langle \nabla^S s, t\rangle + \langle s, \nabla^S t \rangle, and

(2) for all vector fields V, W\in C^{\infty}(TX)\subset C^{\infty}(Cl(TX)) and for any section s\in C^\infty(S):

\nabla^S_V(W\cdot s) = \nabla^X_V W \cdot s + W\cdot \nabla^S_V s.

Definition (A Dirac operator)

A differential operator D: C^\infty(S)\to C^\infty(S) is a Dirac operator if

(1) D is a Dirac type operator, and

(2) S is Clifford compatible.

Lemma

If X is oriented and D is a Dirac operator, then D is formally self-adjoint. That is,
\langle\langle Ds, t\rangle\rangle = \langle\langle s, Dt\rangle\rangle,
where \langle\langle s,t\rangle\rangle = \int_{X} \langle s, t \rangle (and the integration is with respect to the volume form on X).

Proof

Omitted 😦 \square

Consequently we have \text{index}(D)=0 (see Ning’s blog). To make use of the index, then, we introduce a Z_2-grading on Clifford module bundles.

Definition

Recall that a Clifford algebra is \mathbb{Z}-filtered — Cl_n=\cup_{i} {Cl_n^i} with Cl_n^i\cdot Cl_n^j\subset Cl_n^{i+j} — and \mathbb{Z}_2-graded — Cl_n={Cl_n}^{+}\oplus {Cl_n}^- (even and odd products).

A Clifford module bundle S is \mathbf{Z}_2-graded if it decomposes into a direct sum S=S^+\oplus S^- of vector bundles such that for each x\in X and v\in T_xX, one has v\cdot S_x^{\pm} \subset S_x^{\mp}.

Such a \mathbb{Z}_2-graded bundle is compatible if this decomposition is both orthogonal with respect to \, \langle, \rangle and parallel with respect to the covariant derivative \nabla^S, ie. \nabla^S S^{\pm} \subset S^{\pm}.

Example

If X is oriented, the Clifford bundle Cl(TX) = Cl^+(TX) \oplus Cl^-(TX) is a \mathbb{Z}_2-graded compatible Clifford bundle. Some words which may be connected to verify this: Levi Civita connection, induced connection on F(TX), lift to principal spin bundle, induced covariant derivative on associated vector bundle, compatibility with the metric.

Examples of Dirac Operators

We now look at four examples of Dirac operators. The first two are familiar; here we reinterpret them in terms of Clifford modules.

Example 1: The De Rham Operator

Recall that the filtered algebra Cl(V^n,q) = \cup Cl^i(V,q) has associated graded algebra Gr(Cl(V,q))\cong \Lambda^\ast V. (See the Glossary below.)

Lemma

Let f:\Lambda^\ast(\mathbb{R})\to Cl_n(\mathbb{R}) be the map defined by

v_1\wedge \ldots \wedge v_r\mapsto \frac{1}{r!} \sum_{\sigma\in \Sigma_r} \text{sign}(\sigma) v_{\sigma(1)}\cdot v_{\sigma(2)}\cdots v_{\sigma(r)}.

Then f is

(1) an isomorphism of vector spaces

(2) filtration preserving, i.e., f(\oplus_{i\leq r} \Lambda^i(\mathbb{R}))\subset {Cl_n}^r(\mathbb{R}), and

(3) O(n)-equivariant, i.e., f(A\omega) = A f(\omega) for \omega\in \Lambda^\ast(\mathbb{R}) and A\in O(n).

Proof

(1) and (2). Let e_1, \ldots, e_n is an orthonormal basis for \mathbb{R}^n. Since (by an equivalent definition of the exterior algebra) v_1\wedge \ldots \wedge v_r = \frac{1}{r!} \sum_{\sigma\in \Sigma_r} \text{sign}(\sigma) v_{\sigma(1)}\otimes v_{\sigma(2)}\otimes\ldots \otimes v_{\sigma(r)}, we see that f is induced by the map T(\mathbb{R}^n) \to Cl_n(\mathbb{R}) taking e_{i_1}\otimes \cdots\otimes  e_{i_r} to e_{i_1}\ldots e_{e_r} and descending to \Lambda^\ast(V); that is, f(e_{i_1}\wedge\ldots \wedge e_{i_r}) = e_{i_1}\cdots e_{i_r}. It follows that f is an isomorphism and preserves the grading.

(3) Using that Ae_1\wedge \ldots Ae_n = (\text{det} A) (e_1\wedge \ldots e_n), I feel like we need to be working with SO(n) here. Please comment! \square

Corollary

The exterior algebra bundle over a(n oriented?) manifold \Lambda^\ast(T^\ast X) and the clifford bundles Cl(TX) are isomorphic as vector bundles.

Proof

Let F(TX) denote the principal O(n)-bundle associated to TX. By parts (1) and (3) of the lemma, the map f above induces a vector bundle isomorphism \Lambda^\ast(TX) = F(TX)\times_{O(n)} \Lambda^\ast(\mathbb{R}^n) \to F(TX)\times _{O(n)} Cl_n(\mathbb{R}) = Cl(TX). \square

Theorem 2.5.12 (Lawson, Michelsohn)

Under this bundle isomorphism \Lambda^\ast(T^\ast X) \cong Cl(TX), the de Rham operator d+d^\ast corresponds to the Dirac operator D.

Corollary

Since we have already established (see Hailiang’s(?) blog post) that the Euler characteristic \chi(X) of X is equal to \text{Index}(d+d^\ast|_{\Omega^{\text{even}}}:\Omega^{\text{even}}\to \Omega^{\text{odd}}), the theorem (along with the grading-preserving property of f) implies we may also compute it as \text{Index}(D|_{C^\infty(Cl^+(TX))}: C^\infty(Cl^+(TX))\to C^\infty(Cl^-(TX))).

Example 2: The Signature Operator

We now look to reinterpret the signature operator in terms of Clifford bundles.

Recall (see Hailiang’s blog) in the case that n=2p and p is even, the Hodge star operator \ast: \Omega^p\to \Omega^{n-p} is an involution so we can decompose \Omega^p = \Omega^p_+\oplus \Omega^p_- into the +1 and -1 eigenspaces of \ast. We defined the signature operator D = d+d^\ast. Since D\ast = -\ast D, we saw that D took \Omega^p_{\pm} to \Omega^p_{\mp} and letting D_+=D|_{\Omega^p_+}, we found that \text{Index}(D_+) = \text{Sign}(X). The signature \text{sign}(X) of X was defined to be the signature of the quadratic form on \mathcal{H}^p given by (\alpha,\beta)\mapsto \int_X \alpha\wedge \beta.

In the case that p is odd, we had to modify the construction. We complexified, taking \Omega_\mathbb{C}^p = C^\infty(\Lambda^p(T^\ast X\otimes \mathbb{C})), and defined \tau = i\ast. Then the above paragraph went through with \tau replacing \ast and \Omega_\mathbb{C} replacing \Omega.

Let \{e_1, \ldots, e_n\} be an oriented orthonormal basis for \mathbb{R}^n. Let \omega = e_1\cdots e_n in Cl_n. Then by the lemma above and the corresponding properties of the volume form e_1\wedge\ldots \wedge e_n in \Lambda^\ast(\mathbb{R}^n), we obtain that \omega is a basis-independent section of Cl_n(\mathbb{R}).

Lemma

(1) We have \omega^2 = \begin{cases} 1 \qquad n\equiv 0, \, 3 \mod 4\\  -1 \qquad n\equiv 1, \, 2 \mod 4  \end{cases}

(2) If n is even and v\in \mathbb{R}^n, then \omega \cdot v = -v \cdot \omega.

Proof

(1) We compute (e_1\cdots e_n)(e_1\cdots e_n) = (-1)^n(-1)^{n-1}\cdots (-1)^2 (-1)^1 = (-1)^{n(n+1)/2}. Writing n=4k+l for 0\leq l\leq 3, one finds that n(n+1)/2 is even if and only if l=0 or 3.

(2) It suffices to verify for v=e_i. We have (e_1\cdots e_n)\cdot e_i = (-1)^{n-i} e_1\cdots \hat{e}_i\cdots e_n and e_i \cdot (e_1\cdots e_n) = (-1)^i e_1\cdots \hat{e}_i\cdots e_n. (Here, a hat \hat{} indicates that the element be omitted from the product.) Since n is even, (-1)^{n-i}=(-1)^i. \square

Now, \omega acts on any Clifford module via v\mapsto w\cdot v, and by part (1) of the lemma this defines an involution in the case n\equiv 0, 3\mod 4; in the case n\equiv 1, 2\mod 4, i\omega defines an involution. Compare with the Hodge star operator recalled above. So define

\Gamma = \begin{cases} \omega \qquad n\equiv 0, \, 3 \mod 4,\\  i\omega \qquad n\equiv 1, \, 2 \mod 4;  \end{cases}

then \Gamma^2=\text{Id}. Thus if X is an oriented manifold and S is a Clifford module bundle of X, putting S^{\pm} = \{v\in S: \Gamma v = \pm v\}, we have

if n\equiv 0, 3 \mod 4: S = S^+\oplus S^-, or

if n\equiv 1,2 \mod 4: S_\mathbb{C} = S_\mathbb{C}^+\oplus S_\mathbb{C}^-

Corollary

If n is even then S is \mathbb{Z}_2-graded, i.e., for x\in X and v\in T_xX, one has v\cdot S_x^{\pm} \subset S_x^{\mp}.

Proof

By part (2) of the lemma, for v\in T_xX we have \Gamma v = -\Gamma v.
Thus if e\in S_x^{\pm} (i.e. \Gamma e = \pm e) then v\cdot e = v\cdot (\pm \Gamma(e)) = \mp \Gamma(v\cdot e), so v\cdot e\in S^{\mp}. \square

Proposition

If n is even and S is \mathbb{Z}_2-graded compatible, then the associated Dirac operator splits as
D = \left( \begin{array}{cc}0 & D^-\\ D^+ & 0\end{array}\right): C^\infty S^+\oplus C^\infty S^-\to  C^\infty S^+\oplus C^\infty S^-.
In particular, if S=Cl(TX), by Theorem 2.5.12 we have \text{Index}(D^+) = \text{Sign}(X).

Proof

Since S is \mathbb{Z}_2-graded and is compatible (so in particular, the covariant derivative preserves S^{\pm}), the Dirac operator
Ds = \sum e_i\cdot \nabla_{e_i}^S s
takes C^\infty(S^{\pm}) to C^\infty(S^{\mp}). \square

Example 3: twisted Dirac Operators

Preliminary: If S and E are vector bundles over X with covariant derivatives \nabla^S and \nabla^E, respectively, then the tensor product bundle S\otimes E\to X has covariant derivative
\nabla^S\otimes \nabla^E := \text{Id}_S\otimes \nabla^E + \nabla^S\otimes \text{Id}_E.

Fact: If S is a compatible \mathbb{Z}_2-graded Clifford module bundle and (E, g, \nabla^E) is a Riemannian bundle (see Property (1) of a compatible Clifford Bundle above for the definition), then S\otimes E is a compatible \mathbb{Z}_2-graded Clifford module bundle (with Clifford multiplication v\cdot(s\otimes e) = (v\cdot s)\otimes e for v\in Cl_n, s\in S, e\in E). In the case that S=Cl(TX_{\mathbb{C}}), we call the Dirac operator on S\otimes E a twisted Dirac operator.

Fact: (Apparently from topological K-theory) If the Index theorem holds for any twisted Signature operator then it holds for all elliptic differential operators.

Example 4: Spin Manifolds and The Atiyah-Singer Dirac Operator

Recall (see Prasit’s blog) that there is an isomorphism \varphi: Cl_{2l}(\mathbb{C}) \xrightarrow{\cong} M_{2^l}(\mathbb{C}). So since
\mathbb{C}^n is an M_n(\mathbb{C}) module, one has that \mathbb{C}^{2^l} is an Cl_{2l}(\mathbb{C})-module via a\cdot v = \varphi(a)v, for a\in Cl_{2l}(\mathbb{C}) and v\in \mathbb{C}^{2^l}. To avoid confusion, let us call \mathbb{C}^{2^l} with this module structure C.

Now, any M_{2^l}(\mathbb{C})-module is isomorphic to \oplus^{2^l} \mathbb{C}^{2^l}, so it follows that any Cl_{2l}(\mathbb{C})-module is isomorphic to \oplus^{2^l} C.

Let S be a Clifford module bundle. From the above paragraph we see that each fiber (a Cl_{2l}(\mathbb{C})-module) is isomorphic (via \varphi_x, say) to \oplus^{2^l} C_x, where C_x is a copy of C. We may then
ask if this splitting extends over the whole bundle; that is, is there a Clifford module bundle S\to X and a bundle isomorphism \varphi:S\to \oplus^{2^l} C which restricts fiberwise to an isomorphism S_x\cong \oplus^{2^l} C.

In turns out the answer is a resounding “Yes” if X is a spin manifold.

Definition (Spin Structure)

Let E\to X is an N-dimensional vector bundle. A spin structure on E is a principal \text{Spin}(N)-bundle P\to X together with a bundle isomorphism P\times_{\text{Spin}(N)} \mathbb{R}^N\to E. (Then E is the associated vector bundle for P). Using classifying space theory, we may reinterpret this
to say that a spin structure on E is a lift of the classifying map \varphi_E: X\to BGL_N(\mathbb{R}) to B\text{Spin}(N).

We may break up the existence of a spin structure into pieces as follows.

After choosing a metric on E, we may first reduce the structure group of E to O(n). (The only obstruction to doing so is the paracompactness of X.) So we’re left to lift a map \varphi_E: X\to BO(n) to B\text{Spin}(N).

Since B\text{Spin}(N) is the universal cover of BSO(N), we may first try to lift \varphi_E to BSO(n).

The short exact sequence of groups 1\to SO(N)\to O(N)\xrightarrow{\text{det}} \mathbb{Z}_2\to 1 induces a fibration of classifying spaces
X\xrightarrow{\varphi_E} BO(N)\to B\mathbb{Z}_2=K(\mathbb{Z}_2,1).

It turns out that the map X\to BO(N) lifts to BSO(N) if and only if the composite X\xrightarrow{\varphi_E} BO(N)\to B\mathbb{Z}_2=K(\mathbb{Z}_2,1) is nullhomotopic. Since [X, K(\mathbb{Z}_2,1)]\cong H^1(X;\mathbb{Z}_2), there is an element \omega_1(E)\in H^1(X;\mathbb{Z}_2) that vanishes if and only if \varphi_E lifts. We call \omega_1(E) the first Stiefel Whitney class of E.

Similarly, the map X\to BSO(N) lifts to B\text{Spin}(N) if and only if the composition X\to BSO(N)\to B K(\mathbb{Z}_2,1)=K(\mathbb{Z}_2,2) is nullhomotopic; we call the corresponding element in H^2(X;\mathbb{Z}_2)\cong [X, K(\mathbb{Z}_2,2)] (that vanishes iff X\to BSO(N) lifts) the second Stiefel Whitney class \omega_2(E) of E.

Definition (Spin manifold)

We will call an oriented manifold X (so \omega_1(TX)=0) a spin manifold if its tangent bundle TX admits a spin structure (i.e., \omega_2(TX)=0). It can be shown that this is equivalent to the existence of a trivialization of TX over the 2-skeleton of X. (Compare with the fact that X is orientable if and only if TX is trivializable over the 1-skeleton.)

Definition (The Atiyah-Singer Dirac Operator)

Suppose X^n has a spin structure with principal \text{Spin}(n)-bundle P (associated to TX). Since Cl_n(\mathbb{C}) acts on C on the left and \text{Spin}(n)={Cl_{n}}^+(\mathbb{C})\cap \langle S^{n-1}\rangle (where \langle S^{n-1}\rangle has general element x_1\cdot x_2\cdots x_k with x_i\in \mathbb{C}^n, x_i^2=1) is a subgroup of the group of units Cl_n^\times, we may define
the C-bundle associated to P by \mathbf{C} = P\times_{\text{Spin}(n)} C\to X. Since C is a Cl_n(\mathbb{C}) module, \mathbf{C} is a Clifford module bundle.

Some words: By lifting the Levi-Civita connection on P_{SO(n)} one obtains a connection on P_{\text{Spin}(n)}, and hence (see who’s blog?) a covariant derivative \nabla^\mathbf{C} on \mathbf{C} which makes it Clifford compatible
as a \mathbb{Z}_2 graded Clifford module bundle.

We may then define the Atiyah-Singer Dirac Operator by

Ds = \sum e_i\cdot \nabla_{e_i}^\mathbf{C} s.

Some more words:

If Ds=0 then s is called a harmonic spinor.

If X has positive scalar curvature, then D is injective. So if we have ways to compute the index (using the ASHI theorem, for example), we may be able to deduce that X does not admit a metric of positive scalar curvature.

Glossary

(to include links to other blog posts)

Differential Operator (global definition)

If E and F are vector bundles over X (of the same dimension), we define the family of differential operators of order \leq m from E to F by

DO_m(E,F) = \{D\in \text{Hom}_{\mathbb{R}}(C^\infty E, C^\infty F):\, [D,f]\in DO_{m-1}(E,F) \, \forall f\in C^{\infty}(X)\}

with DO_{-1}(E,F)=\{0\}. In particular, DO_{0}(E,F) = \{C^{\infty}(X)\text{-linear bundle maps } C^\infty(E)\to C^\infty(F)\}.

Symbol of a differential operator

(cf. Juanita’s blog) We recall the definition of the symbol \sigma_D of an order-m differential operator D:C^\infty(E)\to C^\infty(F). Denote T^\ast X\xrightarrow{\pi} X, E\xrightarrow{p} X, so \pi^\ast(E)=\{(\xi^\ast,e)\in T^\ast X\times E:\, \pi(\xi^\ast)=x=p(e)\}. Let \xi^\ast\in T^\ast X. Let x\in X. The symbol \sigma_D(\xi^\ast) of D at x is the homomorphism E_x\to F_x defined by

\sigma_D(\xi^\ast)(s(x)) = \frac{1}{i} \frac{1}{m!} D(f^m s)(x)

where s\in C^\infty(E), and f\in C^\infty(X) is such that df_x = \xi^\ast(x).

Covariant Derivative

A covariant derivative \nabla on a vector bundle E\to X is a map \nabla: C^{\infty}(TX)\times C^{\infty}(E)\to C^{\infty}(E), (v,s)\mapsto \nabla^S_v s where \nabla_v^S is an \mathbb{R}-linear map C^\infty(E)\to C^{\infty}(E) satisfying the Leibnitz rule \nabla^S_v (fs) = v(f)s + f\nabla^S_v(s) for s\in C^\infty(E) and f\in C^\infty(X).

Graded Algebras

(cf Prasit’s blog) A k-algebra A is \mathbb{Z}-graded if A=\bigoplus_i A_i such that A_i A_j\subset A_{i+j}. A k-algebra is filtered over \mathbb{Z}
if A=\cup A^i such that A^i\subset A^{i+1} and A^i A^j \subset A^{i+j}. A graded algebra A=\bigoplus_i A_i defines a filtered algebra by taking A^i=A_0\oplus\ldots A_i, and conversely a filtered algebra A=\cup_i A^i defines a graded algebra by taking A_i=A^i/A^{i-1} (with A^{-1}=0).

Tensor Algebra and the Clifford Algebra

If V is a vector space, the tensor algebra TV = \mathbb{R}\oplus V\oplus V\otimes V\oplus\ldots \oplus V\oplus\ldots \oplus V has multiplication defined by concatenation, i.e., (x_1\otimes \ldots \otimes x_j)(y_1\otimes \ldots \otimes y_j) = x_1\otimes \ldots \otimes x_j \otimes y_1\otimes \ldots \otimes y_j. Thus TV is a graded algebra with (TV)_i = \underbrace{V\otimes \ldots \otimes V}_{\text{\emph{i}  times}}, and filtered with (TV)^i=\mathbb{R}\oplus \cdots \oplus  V\otimes\cdots \otimes V.

Recall that Cl(V,q)=\cup_i Cl^i(V,q) with Cl^i(V,q)=(TV)^i/{\langle v\otimes v = -q(v)\rangle}. (Here, q is a quadratic form; sometimes it is convenient to refer instead to the associated symmetric bilinear form \beta.) For any v\in V, q(v)\in \mathbb{R}=Cl^0(V,q) so q(v) maps to 0 under the quotient Gr_i(Cl(V,q))= Cl^i(V,q)/Cl^{i-1}(V,q). This sets up a natural identification between Gr_i(Cl(V,q)) and (\Lambda^\ast V)_i = \frac{(TV)_i}{\langle v\otimes v=0\rangle}. Whence the associated graded algebra for Cl(V,q) is isomorphic to \Lambda^\ast V. In particular, they are isomorphic as vector spaces, with dimension \sum_{k=0}^n \begin{pmatrix}n\\k\end{pmatrix}=2^n.

Clifford bundle

The Clifford bundle Cl(TX)\xrightarrow{\pi} X has fibers Cl(T_xX, g_x) \cong Cl_n(\mathbb{R}) (x\in X), where g is a Riemannian metric on X. (The latter isomorphism is given by identifying an orthonormal (with respect to g_x) basis for T_xX with the standard generators \{e_i\}.) Just as TX is the \mathbb{R}^n-bundle associated to the orthogonal frame bundle (of the tangent bundle) F(TX) over X, Cl(TX) is the associated Cl_n(\mathbb{R})-bundle to F(TX).

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Written by aclightf

November 21, 2012 at 7:45 pm

Posted in Uncategorized

One Response

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  1. We need a third condition to be Clifford compatible. We need that multiplication by an element of S(TX) is an orthogonal transformation of the fiber.

    jfdavis

    December 4, 2012 at 5:38 pm


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