for every closed piecewise C1 curve in D must be holomorphic on D.
The assumption of Morera's theorem is equivalent to that ƒ has an anti-derivative on D.
The converse of the theorem is not true in general. A holomorphic function need not possess an antiderivative on its domain, unless one imposes additional assumptions. The converse does hold e.g. if the domain is simply connected; this is Cauchy's integral theorem, stating that the line integral of a holomorphic function along a closed curve is zero.
There is a relatively elementary proof of the theorem. One constructs an anti-derivative for ƒ explicitly. The theorem then follows from the fact that holomorphic functions are analytic.
Without loss of generality, it can be assumed that D is connected. Fix a point z0 in D, and for any , let be a piecewise C1 curve such that and . Then define the function F to be
To see that the function is well-defined, suppose is another piecewise C1 curve such that and . The curve (i.e. the curve combining with in reverse) is a closed piecewise C1 curve in D. Then,
And it follows that
By continuity of f and the definition of the derivative, we get that F'(z) = f(z). Note that we can apply neither the Fundamental theorem of Calculus nor the mean value theorem since they are only true for real-valued functions.
Since f is the derivative of the holomorphic function F, it is holomorphic. This completes the proof.
Morera's theorem is a standard tool in complex analysis. It is used in almost any argument that involves a non-algebraic construction of a holomorphic function.
for every n, along any closed curve C in the disc. Then the uniform convergence implies that
for every closed curve C, and therefore by Morera's theorem ƒ must be holomorphic. This fact can be used to show that, for any open set Ω ⊆ C, the set A(Ω) of all bounded, analytic functions u : Ω → C is a Banach space with respect to the supremum norm.
Infinite sums and integrals
or the Gamma function
Specifically one shows that
for a suitable closed curve C, by writing
and then using Fubini's theorem to justify changing the order of integration, getting
Then one uses the analyticity of x ↦ xα−1 to conclude that
and hence the double integral above is 0. Similarly, in the case of the zeta function, the M-test justifies interchanging the integral along the closed curve and the sum.
Weakening of hypotheses
The hypotheses of Morera's theorem can be weakened considerably. In particular, it suffices for the integral
to be zero for every closed triangle T contained in the region D. This in fact characterizes holomorphy, i.e. ƒ is holomorphic on D if and only if the above conditions hold.
- Ahlfors, Lars (January 1, 1979), Complex Analysis, International Series in Pure and Applied Mathematics, McGraw-Hill, ISBN 978-0-07-000657-7, Zbl 0395.30001.
- Conway, John B. (1973), Functions of One Complex Variable I, Graduate Texts in Mathematics 11, Springer Verlag, ISBN 978-3-540-90328-4, Zbl 0277.30001.
- Greene, Robert E.; Krantz, Steven G. (2006), Function Theory of One Complex Variable, Graduate Studies in Mathematics 40, American Mathematical Society, ISBN 0-8218-3962-4
- Morera, Giacinto (1886), "Un teorema fondamentale nella teorica delle funzioni di una variabile complessa", Rendiconti del Reale Instituto Lombardo di Scienze e Lettere (in Italian) 19 (2): 304–307, JFM 18.0338.02.
- Rudin, Walter (1987) , Real and Complex Analysis (3rd ed.), McGraw-Hill, pp. xiv+416, ISBN 978-0-07-054234-1, Zbl 0925.00005.