In Stein and Shakarchi, Complex Analysis, Princeton lectures in Analysis, Chapter 2, Problem 2 an interesting question is posed. The problem section in each chapter contains more complicated problems, with a research taste.
Morera's theorem simply states that if a function $f$ is continuous on $\Bbb{C}$ and $\int_D f(z)dz=0$ for any triangle(rectangle) $D$, then $f$ is holomorphic in $\Bbb{C}$. (the theorem is still valid if we replace $\Bbb{C}$ by a disk).
The problem presented above, states that
Morera's theorem is still valid if we replace the contours of integration from triangles/rectangles to circles, and more generally, to any contour which is a translate and dilate of a toy contour $\Gamma$.
Is there a simple proof for this problem, or maybe a reference to an article in which I can find the proofs?
I initially posted it on MO, but I didn't get an answer, and I was told that that was not the place for such questions. I received a comment with an idea of solution, but I don't seem to get it:
Convolve with a mollifier, apply Green, conclude that $\overline{\partial}$ of the convolution is $0$, recall that the uniform limit of analytic functions is analytic.
I would be glad if you could explain a bit how the answer above works for solving the initial problem.