Several posts discuss the representation of the delta function in polar coordinates in 2D or 3D, e.g. Dirac delta in polar coordinates or Delta function at the origin in polar coordinates
Does anyone have a reference for a representation of $\delta(|\mathbf x|)$ or $\delta(r)$ in general dimension $n \geq 2$? I am guessing the following: $$ \delta(r)/r^{n-1} = s_{n-1} \delta(\mathbf x), \qquad s_{n-1} := {2\pi^{n/2} \over \Gamma(n/2)} \text{ (area of unit sphere)} $$ or equivalently (?) $$ \delta(|\mathbf x|) = s_{n-1} |\mathbf x|^{n-1} \delta(\mathbf x) $$
My proof: For $r_0 \neq 0$ the general curvilinear correspondence is $$ \delta(\mathbf {x - x}_0) \equiv \delta(r - r_0) \delta(\mathbf u - \mathbf u_0) / r^{n-1}, \qquad \mathbf {x, x}_0\in\mathbb R^n\quad r,r_0>0\quad \mathbf{u, u}_0\in\mathbb S^{n-1} \text{ (unit sphere)} $$ The case $\mathbf x_0 = \mathbf 0 \Leftrightarrow (r_0 = 0, \mathbf u_0\in\mathbb S^{n-1})$ corresponds to a singular Jacobian of the transformation and we may "integrate out" all ignorable spherical coordinates $\mathbf u$: $$ \begin{aligned}\delta(\mathbf x) \int_{\mathbb S^{n-1}} d\mathbf u & = \delta(r) / r^{n-1} \int_{\mathbb S^{n-1}} \delta(\mathbf u - \mathbf u_0) d\mathbf u, \qquad\text{i.e.} \\ s_{n-1} \delta(\mathbf x) & = \delta(r) / r^{n-1} \end{aligned} $$ as required.