I was told that the formal adjoint of the gradient is the negative divergence.
Let $A : H\to H$ be a bounded, linear operator, The adjoint of $A$, i.e. $A^*: H\to H$ satisfies \begin{equation*} (Au,v)=(u,A^*v) \end{equation*} for all $u, v\in H$. Moreover, $A$ is symmetric if $A^*=A$.
Could anyone help me to verify the claim:
what would be the adjoint of the gradient in the space $L^2(\mathbb{R}^d)$?