## The Goldman bracket

I was in Stony Brook last week, visiting Moira Chas and Dennis Sullivan, and have been away from blogging for a while; this week I plan to write a few posts about some of the things I discussed with Moira and Dennis. This is an introductory post about the Goldman bracket, an extraordinary mathematical object made out of the combinatorics of immersed curves on surfaces. I don’t have anything original to say about this object, but for my own benefit I thought I would try to explain what it is, and why Goldman was interested in it.

In his study of symplectic structures on character varieties $\text{Hom}(\pi,G)/G$, where $\pi$ is the fundamental group of a closed oriented surface and $G$ is a Lie group satisfying certain (quite general) conditions, Bill Goldman discovered a remarkable Lie algebra structure on the free abelian group generated by conjugacy classes in $\pi$. Let $\hat{\pi}$ denote the set of homotopy classes of closed oriented curves on $S$, where $S$ is itself a compact oriented surface, and let $\mathbb{Z}\hat{\pi}$ denote the free abelian group with generating set $\hat{\pi}$. If $\alpha,\beta$ are immersed oriented closed curves which intersect transversely (i.e. in double points), define the formal sum

$[\alpha,\beta] = \sum_{p \in \alpha \cap \beta} \epsilon(p; \alpha,\beta) |\alpha_p\beta_p| \in \mathbb{Z}\hat{\pi}$

In this formula, $\alpha_p,\beta_p$ are $\alpha,\beta$ thought of as based loops at the point $p$, $\alpha_p\beta_p$ represents their product in $\pi_1(S,p)$, and $|\alpha_p\beta_p|$ represents the resulting conjugacy class in $\pi$. Moreover, $\epsilon(p;\alpha,\beta) = \pm 1$ is the oriented intersection number of $\alpha$ and $\beta$ at $p$.

This operation turns out to depend only on the free homotopy classes of $\alpha$ and $\beta$, and extends by linearity to a bilinear map $[\cdot,\cdot]:\mathbb{Z}\hat{\pi} \times \mathbb{Z}\hat{\pi} \to \mathbb{Z}\hat{\pi}$. Goldman shows that this bracket makes $\mathbb{Z}\hat{\pi}$ into a Lie algebra over $\mathbb{Z}$, and that there are natural Lie algebra homomorphisms from $\mathbb{Z}\hat{\pi}$ to the Lie algebra of functions on $\text{Hom}(\pi,G)/G$ with its Poisson bracket.

The connection with character varieties can be summarized as follows. Let $f:G \to \mathbb{R}$ be a (smooth) class function (i.e. a function which is constant on conjugacy classes) on a Lie group $G$. Define the variation function $F:G \to \mathfrak{g}$ by the formula

$\langle F(A),X\rangle = \frac {d}{dt}|_{t=0} f(A\text{exp}{tX})$

where $\langle \cdot,\cdot\rangle$ is some (fixed) $\text{Ad}$-invariant orthogonal structure on the Lie algebra $\mathfrak{g}$ (for example, if $G$ is reductive (eg if $G$ is semisimple), one can take $\langle X,Y\rangle = \text{tr}(XY)$). The tangent space to the character variety $\text{Hom}(\pi,G)/G$ at $\phi$ is the first cohomology group of $\pi$ with coefficients in $\mathfrak{g}$, thought of as a $G$ module with the $\text{Ad}$ action, and then as a $\pi$ module by the representation $\phi$. Cup product and the pairing $\langle\cdot,\cdot\rangle$ determine a pairing

$H^1(\pi,\mathfrak{g})\times H^1(\pi,\mathfrak{g}) \to H^2(\pi,\mathbb{R}) = \mathbb{R}$

where the last equality uses the fact that $\pi$ is a closed surface group; this pairing defines the symplectic structure on $\text{Hom}(\pi,G)/G$.

Every element $\alpha \in \pi$ determines a function $f_\alpha:\text{Hom}(\pi,G)/G \to \mathbb{R}$ by sending a (conjugacy class of) representation $[\phi]$ to $f(\phi(\alpha))$. Note that $f_\alpha$ only depends on the conjugacy class of $\alpha$ in $\pi$. It is natural to ask: what is the Hamiltonian flow on $\text{Hom}(\pi,G)/G$ generated by the function $f_\alpha$? It turns out that when $\alpha$ is a simple closed curve, it is very easy to describe this Hamiltonian flow. If $\alpha$ is nonseparating, then define a flow $\psi_t$ by $\psi_t\phi(\gamma)=\phi(\gamma)$ when $\gamma$ is represented by a curve disjoint from $\alpha$, and $\psi_t\phi(\gamma)= \text{exp} tF_\alpha(\phi)\phi(\gamma)$ if $\gamma$ intersects $\alpha$ exactly once with a positive orientation (there is a similar formula when $\alpha$ is separating). In other words, the representation is constant on the fundamental group of the surface “cut open” along the curve $\alpha$, and only deforms in the way the two conjugacy classes of $\alpha$ in the cut open surface are identified in $\pi$.

In the important motivating case that $G = \text{PSL}(2,\mathbb{R})$, so that one component of $\text{Hom}(\pi,G)/G$ is the Teichmüller space of hyperbolic structures on the surface $S$, one can take $f = 2\cosh^{-1}\text{tr/2}$, and then $f_\alpha$ is just the length of the geodesic in the free homotopy class of $\alpha$, in the hyperbolic structure on $S$ associated to a representation. In this case, the symplectic structure on the character variety restricts to the Weil-Petersson symplectic structure on Teichmüller space, and the Hamiltonian flow associated to the length function $f_\alpha$ is a family of Fenchel-Nielsen twists, i.e. the deformations of the hyperbolic structure obtained by cutting along the geodesic $\alpha$, rotating through some angle, and regluing. This latter observation recovers a famous theorem of Wolpert, connected in an obvious way to his formula for the symplectic form $\omega = \sum dl_\alpha \wedge d\theta_\alpha$ where $\theta$ is angle and $l$ is length, and the sum is taken over a maximal system of disjoint essential simple curves $\alpha$ for the surface $S$.

The combinatorial nature of the Goldman bracket suggests that it might have applications in combinatorial group theory. Turaev discovered a Lie cobracket on $\mathbb{Z}\hat{\pi}$, and showed that together with the Goldman bracket, one obtains a Lie bialgebra. Motivated by Stallings’ reformulation of the Poincaré conjecture in terms of group theory, Turaev asked whether a free homotopy class contains a power of a simple curve if and only if the cobracket of the class is zero. The answer to this question is negative, as shown by Chas; on the other hand, Chas and Krongold showed that a class $\alpha$ is simple if and only if $[\alpha,\alpha^3]$ is zero. Nevertheless, the full geometric meaning of the Goldman bracket remains mysterious, and a topic worthy of investigation.

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### 5 Responses to The Goldman bracket

1. Andy P. says:

There’s an interesting earlier (ie 1978) paper of
Turaev entitled “Intersections of loops in
two-dimensional manifolds”. In that paper, he gives
an intersection pairing on $\pi_1$ of a surface (with
one boundary component; the basepoint is on the boundary) that
is superficially very similar to the Goldman bracket,
but actually has rather different, intriguing properties.

It is a pairing
$\Z \pi_1 \times \Z \pi_1 \rightarrow \Z \pi_1$.
Unlike the Goldman bracket, its values depend strongly on
the basepoint. However, it interacts very well with
the group-theory of the surface group. In particular,
it is actually a biderivation in an appropriate sense. Even
more interestingly, I noticed when I read Turaev’s paper that
it interacts well with the derived series of $\pi_1$. Let
$S_k$ be the quotient of $\pi_1$ by the kth term
of its derived series, so $\latex S_k$ is a k-step solvable
group. Then Turaev’s bracket descends to a pairing
$\Z S_k \times \Z S_k \rightarrow \Z S_{k-1}$. For $k=1$,
we have $S_{k-1} = 1$ and this is just the algebraic intersection
pairing. For the higher terms, however, we get something new. In
particular, for $k=2$ we have $S_{k-1}$ equal to the
first homology group of the surface, so we get an “intersection pairing”
on the two-step solvable truncation of $\pi_1$ with values in
the group ring of its first homology group!

I’ve always thought that this should have nice applications
(maybe to understanding the “solvable” version of the Johnson
filtration of the mapping class group), but I haven’t yet managed
to find any…

2. Andy P. says:

Let’s try that again and see if I can get the formulas to parse.

There’s an interesting earlier (ie 1978) paper of
Turaev entitled “Intersections of loops in
two-dimensional manifolds”. In that paper, he gives
an intersection pairing on $\pi_1$ of a surface (with
one boundary component; the basepoint is on the boundary) that
is superficially very similar to the Goldman bracket,
but actually has rather different, intriguing properties.

It is a pairing
$\mathbb{Z} \pi_1 \times \mathbb{Z} \pi_1 \rightarrow \mathbb{Z} \pi_1$.
Unlike the Goldman bracket, its values depend strongly on
the basepoint. However, it interacts very well with
the group-theory of the surface group. In particular,
it is actually a biderivation in an appropriate sense. Even
more interestingly, I noticed when I read Turaev’s paper that
it interacts well with the derived series of $\pi_1$. Let
$S_k$ be the quotient of $\pi_1$ by the kth term
of its derived series, so $S_k$ is a k-step solvable
group. Then Turaev’s bracket descends to a pairing
$\mathbb{Z} S_k \times \mathbb{Z} S_k \rightarrow \mathbb{Z} S_{k-1}$. For $k=1$,
we have $S_{k-1} = 1$ and this is just the algebraic intersection
pairing. For the higher terms, however, we get something new. In
particular, for $k=2$ we have $S_{k-1}$ equal to the
first homology group of the surface, so we get an “intersection pairing”
on the two-step solvable truncation of $\pi_1$ with values in
the group ring of its first homology group!

I’ve always thought that this should have nice applications
(maybe to understanding the “solvable” version of the Johnson
filtration of the mapping class group), but I haven’t yet managed
to find any…

• Danny Calegari says:

Hi Andy – thanks for the comment and the reference! I was not aware of this paper (I guess it is this one), although I knew in a general way that Turaev has done several interesting things with the combinatorics of immersed curves on surfaces. An endlessly mysterious and fascinating subject!

3. Peter says:

Hi Danny,

I’ve been reading about the Goldman form a little bit recently and I found this blog post, which was very nice to read. I had what I assume is a stupid question: In a couple of his papers Goldman restricts to working with closed surfaces of genus g > 1. Do you know why he doesn’t include the torus? My impression is that it has something to do with $\pi_1$ being abelian, but I don’t know what. Do you know if he has this restriction because the symplectic form (or corresponding Poisson bracket) don’t exist, or because the character variety isn’t as well-behaved?

A paper that I thought was interesting that uses this Poisson bracket is http://www.ams.org/mathscinet/search/publdoc.html?pg1=MR&s1=1691437&loc=fromreflist
A one-sentence summary is that the “deformation quantization of the ring of functions on the character variety of $\pi_1$ of a surface” has a natural geometric interpretation.

Peter
p.s. I was a student in your class on complex curves a few years back (winter of 04-05?), which was a nice class :-)

• Danny Calegari says:

Hi Peter – as you say, the issue is with pi_1 being abelian, so there are no “interesting” representations. Although even in this case, if one thinks of H^1 as the “character variety” of representations of pi_1 to R, there is a symplectic form on this space coming from the intersection pairing. At least I think this is why Goldman ignores this case . . .

best,

Danny

(ps I haven’t updated my blog in a long time; I keep intending to get around to it though)