 Weak derivative

A locally integrable function $g$ is the $i$th weak derivative of $u$ whenfor every test function $\phi \in C_c^{\infty }(U)$. Thus a weak derivative is a distributional derivative that is represented by a locally integrable function.

A weak derivative is a concept used in the field of mathematical analysis, particularly in the study of Sobolev spaces. It generalizes the idea of a derivative to functions that may not be differentiable in the classical sense but are still "nice" enough for analysis. The key idea behind weak derivatives is to allow for the differentiation of functions that may have discontinuities or other irregularities.