# Morera's Theorem

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## Theorem

Let $D$ be a simply connected domain in $\C$.

Let $f: D \to \C$ be a continuous function.

Let $f$ be such that:

- $\ds \int_\gamma \map f z \rd z = 0$

for every simple closed contour $\gamma$ in $D$

Then $f$ is analytic on $D$.

## Proof

For a fixed $z_0 \in D$ and $z \in D$ we consider the function:

- $\ds \map F z = \int_\gamma \map f w \rd w$

where $\gamma$ is any (simple) contour starting at $z_0$ and ending at $z$.

By Primitive of Function on Connected Domain, $F$ is a primitive of $f$.

Since $F$ is analytic and $F' = f$, we conclude that $f$ is analytic as well.

$\blacksquare$

## Also see

This is the converse of the Cauchy-Goursat Theorem.

## Source of Name

This entry was named for Giacinto Morera.

## Sources

- 1977: Serge Lang:
*Complex Analysis*