# Doubt in the formulation of Cauchy's integral formula

I'm reading Conway's complex analysis book and on page 84 he says:

Quite often I see this version of the theorem:

Let $f$ be analytic in the simply connected domain $D$ and let $C$ be a simple closed positively oriented contour that lies in $D$. If $z_0$ is a point that lies interior to $C$, then $$f(z_0)=\frac{1}{2\pi i}\int_C\frac{f(z)}{z-z_0}$$ (From this book page 235)

The Conway's formulation seems more general. The problem is I can't understand why Conway's formulation implies in the second formulation. Is the winding number in the case of the second formulation one?

• That the winding number is one, is exactly en.wikipedia.org/wiki/Jordan_curve_theorem (as $z_0$ is in the interior). And this is the reason why the first theorem implies the second – b00n heT Aug 29 '16 at 13:50
• @b00nheT but the winding number is zero when $\gamma$ is homotopic to a constant, no? – user42912 Aug 29 '16 at 19:02
• One can derive the first theorem without the knowledge of Jordan curve theorem & the theorem that says that the winding number around Jordan curve is $\pm 1$. But note: there is a different definition for "simply connected domain" that considers homotopic curves and not Jordan curves, i.e. simply connected domain is a region where we can deform any loop into a point. But if you know the Jordan curve theorem, then you can immediately define simple connected domain without any hocus pocus... – Hulkster Aug 29 '16 at 19:38
• @Juho Please could you explain the relationship between my doubt and the Jordan Curve theorem. I didn't understand what this theorem has to do with everything else. Thank you – user42912 Aug 29 '16 at 22:28
• Note also that you can't always deform loop $\gamma$ into a point without crossing the point $a$. – Hulkster Aug 29 '16 at 23:33

The winding number may in fact be 'defined' as $$n(\gamma;a)= \frac{1}{2\pi i} \oint_\gamma \frac{1}{z-a} \; dz$$ and this is typically also used in the proof. In Conwey's version $G$ need not be simply connected, $\gamma$ may intersect itself, but $\gamma$ should not effectively 'wind around' any points in the complement of $G$. The winding number is indeed one in the second formulation.
• But if $\gamma$ doesn't "wind around" then the winding number is zero, no? – user42912 Aug 29 '16 at 14:04
• Indeed. When $\gamma$ is the boundary of an open domain $U$ and goes around counter-clockwise (this corresponds to a curve $C$ as in the second formulation) then $n(\gamma;a)=+1$ if $a\in U$ and zero otherwise. Conwey's formulation allows for the curve $\gamma$ to wind around and then unwind again. For example, if you have an annulus the geometric image of $\gamma$ may in fact encircle the hole in the middle! but still have winding number 0 around points in the hole (you should probably make a drawing for this). – H. H. Rugh Aug 29 '16 at 14:12