quantum field theory - Multivariable Dirac Delta and Faddeev-Popov Determinant

From this mathstack page and in particular Qmechanic's answer:

1. There exists an $n$-dimensional generalization $$\tag{1} \delta^n({\bf f}({\bf x})) ~=~\sum_{{\bf x}_{(0)}}^{{\bf f}({\bf x}_{(0)})=0}\frac{1}{|\det\frac{\partial {\bf f}({\bf x})}{\partial {\bf x}} |}\delta^n({\bf x}-{\bf x}_{(0)})$$ of the substitution formula for the Dirac delta distribution under pertinent assumptions, such as e.g., that the function ${\bf f}:\Omega \subseteq \mathbb{R}^n \to \mathbb{R}^n$ has isolated zeros. Here the sum on the rhs. of eq. (1) extends to all the zeros ${\bf x}_{(0)}$ of the function ${\bf f}$.

Also from this page on the Faddeev-Popov procedure they say:

For ordinary functions, a property of the Dirac delta function gives: $\delta(x-x_0) = \left|\frac{df(x)}{dx}\right|_{x=x_0}\delta(f(x))\,$ assuming $f(x)\,$ has only one zero at $x=x_0\,$ and is differentiable there. Integrating both sides gives :

$$1 = \left|\frac{df(x)}{dx}\right|_{x=x_0}\int\!dx\,\delta(f(x))\,$$ . Extending over $n$ variables, suppose $f(x^i) = 0\,$ for some $x^i_0\,$. Then, replacing $\delta(x-x_0)\,$ with $\prod_i^n \delta^i(x^i-x^i_0)\,$ : $$1 = \left(\prod_i \left|\frac{\partial f(x^i)}{\partial x^i}\right|\right) \int\!\left(\prod_i dx^i\right)\,\delta(f(x^i))\,$$ . Recognizing the first factor as the determinant of the diagonal matrix $\frac{\partial f(x^i)}{\partial x^i}\delta^{ij}\,$ (no summation implied), we can generalize to the functional version of the identity: : $$1 = \det\left|\frac{\delta G}{\delta \Omega}\right|_{G=0} \int\!\mathcal{D}\Omega\,\delta[G_a(\phi^\Omega)]\,$$ , where $\Delta_F[\phi] \equiv \det\left|\frac{\delta F}{\delta g}\right|_{F=0}\,$ is the Faddeev-Popov determinant.

What I don't understand is that it seems their function $f$ seems to be $f:\Omega \subseteq \mathbb{R}^n \to \mathbb{R}$. How does the generalized Dirac formula $(1)$ work in this case? I don't really understad their notation in:

$$1 = \left(\prod_i \left|\frac{\partial f(x^i)}{\partial x^i}\right|\right) \int\!\left(\prod_i dx^i\right)\,\delta(f(x^i))\,$$

What does

$$\frac{\partial f(x^i)}{\partial x^i}$$ mean here?

Answer

The notation

$$\begin{equation} \frac{ \partial f_i}{ \partial x ^i } \end{equation}$$ means the diagonal elements of the matrix: $$\begin{equation} J _{ ij} = \frac{ \partial f _i }{ \partial x ^j } \end{equation}$$ where $f_i$ is the component of the vector $\vec{f} (x)$.

---

I found this very confusing a few weeks ago so. Here is the proof I wrote up for the identity based on the response I received to an earlier question of mine here:

Recall that if $f (x)$ has one zero at $x _0$ then,

$$\begin{equation} \int d x \left| \frac{ df (x) }{ d x } \right| _{ x = x _0 } \delta \left( f (x) \right) = 1 \end{equation}$$ We want to generalize this to instead of having $f (x)$ we have, ${\mathbf{g}} ( {\mathbf{a}} )$ for vectors of arbitrary size. To do this consider the Taylor expansion of ${\mathbf{g}}$ around its root (we assume it only has one root, ${\mathbf{a}} _0$): $$\begin{equation} g _i ( {\mathbf{a}} ) = \overbrace{g _i ( {\mathbf{a}} _0 )}^0 + \sum _{ j} \frac{ \partial g _i }{ \partial a _j } \bigg|_{ a _0 } ( a _j - a _{ 0,j }) + ... \end{equation}$$ We want to insert this into a delta function, $\delta ^{ ( n ) } ( {\mathbf{g}} ( {\mathbf{a}} ) )$. This will only be nonzero near ${\mathbf{a}} = {\mathbf{a}} _0$. Thus we have, $$\begin{align} \delta \left( {\mathbf{g}} ( {\mathbf{a}} ) \right) & = \prod _i \delta \left( g _i ( {\mathbf{a}} ) \right) \\ & = \prod _i \delta \big( \sum _j J _{ ij} ( a _j - a _{ 0,j} ) \big) \end{align}$$ where $J _{ ij}$ is the Jacobian matrix defined by $J _{ ij} \equiv \frac{ \partial g _{ i} }{ \partial a _j } \big|_{ a _0 }$. We have, $$\begin{align} \delta \left( {\mathbf{g}} ( {\mathbf{a}} ) \right) & = \delta \big( \sum _j J _{ 1j} ( a _j - a _{ 0,j} ) \big) \delta \big( \sum _j J _{ 2j} ( a _j - a _{ 0,j} ) \big) ... \end{align}$$ We now use the identity, $$\begin{equation} \delta ( \alpha ( a - a _0 ) ) = \frac{ \delta ( a - a _0 ) }{ \left| \alpha \right| } \end{equation}$$ We choose to isolate each delta function in the equation above for a different $a _j$: $$\begin{align} \delta \big( {\mathbf{g}} ( {\mathbf{a}} ) \big) & = \frac{ \delta ( a _1 - a _{ 0,1 } ) }{ \left| J _{ 1,1 } \right| } \frac{ \delta ( a _2 - a _{ 0,2 } ) }{ \left| J _{ 2,2 } \right| } ... \end{align}$$ If we take the Jacobian matrix to be greater then zero then we have the product: $$\begin{equation} ( J _{ 1,1 } J _{ 2,2} .. ) ^{-1} = \frac{1}{ \det J } \end{equation}$$ where we have used the fact that the determinant of $J$ is independent of a unitary transformation. So we finally have, $$\begin{align} \left( \int \prod _{ i} d a _i \right) \delta ^{ ( n ) } \big( {\mathbf{g}} ( {\mathbf{a}} ) \big) \det \big( \frac{ \partial g _i }{ \partial a _j } \big) & = 1 \end{align}$$ where it is understood that the Jacobian matrix is evaluated at the root of ${\mathbf{g}}$.

We write the continuum generalization of this equation as,

$$\begin{equation} \int {\cal D} \alpha (x) \delta \left( G ( A ^\alpha ) \right) \det \left( \frac{ \delta G ( A ^\alpha ) }{ \delta \alpha } \right) = 1 \end{equation}$$