# What is a reference for the ( classical and well-known ) proof of Weyl's lemma?

What is a reference for the (classical and well-known) proof of Weyl's lemma that states:

Let $U$ be an open subset of $R^n$. Then if $f\in L^1_{loc} (U)$ and if $\int_U \hspace{1mm}{f\phi_\bar{z}}=0\;\;\;\forall \phi \in C_c^{\infty}(U)$, then $f$ is a.e. equal to a holomorphic function.

Just any quick and good reference would be appreciated. I know Weyl's lemma has a weaker form involving weak Laplacian. Where can I find a proof of that?

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This is a special case of elliptic regularity (en.wikipedia.org/wiki/Elliptic_regularity), which implies that $f$ is smooth and hence must satisfy the Cauchy-Riemann equations. So a book on PDE (e.g. Evans) should have it. –  Akhil Mathew May 24 '11 at 22:29