# Delta function and integrating over level sets?

Consider the three-dimensional integral $$\int_{\mathbb R^3} d^3x\,f(x)\delta(g(x))$$ where $\delta$ is the dirac delta, $f,b:\mathbb R^3\to\mathbb R$ and $g(x) = 0$ on some surface $S$. Is there a way of rewriting the above integral as a surface integral over the level set $S$? Related to this, is there some distributional identity like $$\delta(g(x)) = \int_S ds dt \frac{\delta^{(3)}(x - X(s,t))}{|g'(X(s,t))|}$$ where $X(s,t)$ is a parameterization of $S$ that would allow one to to this, analogous to the formula $$\delta(h(x)) = \sum_{x_0\in h^{-1}(0)}\frac{\delta^{(3)}(x-x_0)}{|h'(x_0)|}$$ when the zero set of $h$ is finite?

-

## This question has an open bounty worth +50 reputation from Dave ending in 4 days.

The question is widely applicable to a large audience. A detailed canonical answer is required to address all the concerns.

Important for a coherent, geometric interpretation of scattering integrals in Quantum Field Theory.