In mathematics, a function ${\displaystyle f}$ is weakly harmonic in a domain ${\displaystyle D}$ if

${\displaystyle \int _{D}f\,\Delta g=0}$

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for all ${\displaystyle g}$ with compact support in ${\displaystyle D}$ and continuous second derivatives, where Δ is the Laplacian.^[1] This is the same notion as a weak derivative, however, a function can have a weak derivative and not be differentiable. In this case, we have the somewhat surprising result that a function is weakly harmonic if and only if it is harmonic. Thus weakly harmonic is actually equivalent to the seemingly stronger harmonic condition.

• Weak solution
• Weyl's lemma