The Duistermaat Heckman localization formula states how integrals over symplectic spaces with Hamiltonian $U(1)$ group actions.
$$ \int_M \frac{\omega^n}{n!} e^{-\mu} = \sum_{x_i \text{ fixed}} \frac{e^{-\mu(x_i)}}{e(x_i)} $$
Here $M$ is a symplectic manifold and here are a few invariants:
- $\omega$ is the symplectic form
- $\mu$ is the moment map of the $U(1)$ rotation action
- $x_i$ are the fixed points of the group action
- $e(x_i)$ is the product of the weights of the rotation at fixed point $x_i$
In a few cases, we can produce formulas an engineer might recognize:
- $M = \mathbb{R}^2 \simeq \mathbb{C}$ and $\omega = dx \wedge dy$
- $U(1)$ the the rotation $z = x_1 + i x_2 \mapsto e^{i\theta}z$, the moment map is $\mu(z) = |z|^2 = x_1^2 + x_2^2$
- Certainly $\mathbb{R}^{2n}$ is analogous.
The fixed point of the rotation is $x_i = 0$. This leads to the formula for the Gaussian integral:
$$ \int (dx \wedge dy) \,e^{-(x^2 + y^2)} = \frac{e^{-\mu(0)}}{e(0)} = \frac{1}{2\pi}$$
I'm not actually sure that $e(0) = 2\pi$ but I'm guessing. There's not too many cases were all the ingredients are known. It is possible to write a manifold (or orbifold or variety) with moment map $\mu$.
My understanding is that we can rarely write $\omega$ down explicitly. For the sphere $S^2 = \{ x^2 + y^2 + z^2 = 1\}$ there's a symplectic form $\omega = \frac{dx \wedge dy}{z}$ which is invariant under rotating aroun the $z$ axis.
Are there other cases where the DH formula specializes in a particularly nice way? I'm guessing that typically we bypass explicitly computing $\omega$.
