# Residue theorem with winding numbers

In my studies, I've been uniquely using the Residue Theorem with no more than a single winding around each singularity. Actually, my professor has never mentioned winding-numbers in the Residue Theorem.

I've been reading discussions about complex integrals, where some people suggest solutions that take winding numbers into account. Is there a general advantage in doing so?

## migrated from mathoverflow.netJan 31 at 18:02

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I taught a basic complex analysis class 16 times so far. Initially I did not include the winding number, and restricted the residue theorem to simple closed curves. This is visually satisfactory and usually sufficient for applications, but it gives a hard time for the teacher and the careful student alike. The reason is that one needs to talk about "the interior" and "counterclockwise orientation" of a simple closed curve. However, I found that discussing these concepts rigorously is beyond an introductory course. So, after a while, I decided to include the winding number: it simplified the proofs and made the theorem more general. Here is the version I currently teach in my course:

Residue Theorem. Let $$M\subset\mathbb{C}$$ be open. Let $$\gamma\subset M$$ be a closed curve freely homotopic to a constant curve within $$M$$. Assume that $$f$$ is holomorphic on $$M\setminus S$$, where $$S\subset M$$ is a discrete subset of isolated singularities of $$f$$ disjoint from $$\gamma$$. Then $$\int_\gamma f=2\pi i\sum_{p\in S} w_p(\gamma)\mathrm{res}_p(f),$$ where the winding number $$w_p(\gamma)$$ vanishes for all but finitely many points $$p\in S$$.

• What does freely homotopic mean? – Vít Tuček Jan 31 at 17:02
• @VítTuček: see en.wikipedia.org/wiki/Free_loop – Ben McKay Jan 31 at 17:17
• @VítTuček: Free homotopy is homotopy without a base point (see e.g. proofwiki.org/wiki/Definition:Homotopy/Free) Fun fact: it took me several years of teaching complex analysis before I learned the proper terminology. For a while I just told my students "homotopy without a base point". – GH from MO Jan 31 at 17:41
• That's interesting. I've studied complex analysis as part of a course in "advanced mathematics for engineers", where homotopy is never once mentioned. It's nice to extend my knowledge. What I was really wondering is whether winding numbers may help in the evaluation of integrals via the residue theorem. – Minamoto Yoshitsune Feb 1 at 16:20