Guide to Cultivating Complex Analysis: Changes

July 9th 2022 edition (version 1.3):

The main point of this revision was to go through the exercises and shake out as many typos as possible, especially for the exercises that weren't assigned in my class. The changes are all very minor in character.

1. In the proof of Proposition 3.1.8, mark which integral is the arclength integral of the modulus.
2. In Exercise 3.1.13 be more explicit either the path is injective or it is simple closed. The way it was stated made it seem like plausibly one that just bites itself somewhere is also allowed, which was not intended.
3. In the intro to 3.3.2, the example of real differentiable function would only work for bounded $$g$$ so integrate from some $$c \in (a,b).$$
4. In proof of Theorem 3.3.6, maximum modulus principle, consider a closed disc in $$U$$ rather than just saying a circle, it is clearer this way, we are talking about $$|z| \leq r$$ anyhow here.
5. Streamline the wording of Exercise 3.3.19 a bit and include the definition of $$\limsup_{z \to \infty} .$$
6. Change the wording of Theorem 3.3.10 (Liouville) to be more precise.
7. Improve the wording of proof of Theorem 3.3.11 (FTA) a bit more.
8. In the proof of Schwarz's lemma, talk about maximum modulus getting the bound $$|g(z)| \leq \frac{1}{r}$$ instead of $$r|f(z)| \leq |z| .$$ Then only once we show that $$|g(z)| \leq 1$$ go to $$r|f(z)| \leq |z| .$$ Seems more straightforward stated this way.
9. Note that Exercise 3.5.3 is called the Cartan's uniqueness theorem and add it to the index.
10. In exercise 3.5.7, ask for the condition $$ad-bc > 0$$ rather than just $$ad-bc \not= 0 .$$ While correct that was misleading, and was not what was intended.
11. In proof of Corollary 5.2.3, emphasize where proof of first item ends and the proof of the converse statement starts.
12. In the comments after Definition 5.2.4, emphasize that $$\ell \in \mathbb{Z} .$$
13. Reword Exercise 5.2.23 to be a bit more readable.
14. Reword Exercise 5.4.6 to make it explicit as to what the power sums are.
15. Reword proof of Exercise 5.4.7, it is a bit misleading.
16. Simplify statement of Hurwitz a tiny bit.
17. In Exercise 6.2.2, emphasize that $$U \subset {\mathbb{C}},$$ as the two notions are not the same in an arbitrary metric space.
18. Reword part a) of 6.2.6 to be more explicit.
19. In Theorem 7.4.5 (maximum principle for subharmonic functions), add a footnote to draw attention to the maximum now being a global one.
20. Reword parts b and c of Exercise 9.2.7 to be more logical.
22. Many other minor clarifications and cleanups.
23. Fixed many misspellings, grammar and style issues.
24. Fix the known errata.

May 10th 2022 edition (version 1.2):

The changes are all quite minor, though numerous. The main focus was to weed out any errata, and improve unclear wording. No new content, although there are three new exercises.

1. Add polar form and polar coordinates to the index.
2. On page 18, improve figure 1.4 to include a shaded horizontal and vertical strip in two shades of gray to make it easier to see.
3. Also on page 18, use $$w$$ for the target variable to avoid confusion when defining the annulus and the sector.
4. At end of section 1.3, when discussing arithmetic, note that $$z+\infty=\infty$$ can be defined, it is just that $$\infty+\infty$$ and $$\infty-\infty$$ are undefined.
5. In section 1.4, when defining $$T_a$$ and $$D_a$$ note where $$a$$ lives, in particular, that for $$D_a,$$ the $$a$$ should be nonzero.
6. In subsection 2.1.1 emphasize a tiny bit more the fact that the $$h$$ is complex.
7. In subsection 2.1.1 explicitly state what $$o(|h|)$$ means rather than being vague.
8. Reword the paragraph in front of Proposition 2.1.4 to be clearer.
9. Add the value of $$f'(z_0)$$ in Proposition 2.1.4 in terms of the real partials as we have also proved that above, and it is good to emphasize.
10. At end of subsection 2.1.2 emphasize the exercise for holomorphicity of the exponential.
11. In the first proof of Proposition 2.2.2, handle the $$k=0$$ case explicitly.
12. In second the proof of Proposition 2.2.2, don't refer to $$f$$ and $$g$$ being holomorphic, they are simply complex differentiable at one point.
13. Add some segue sentences in subsection 2.2.1, and rename Proposition 2.2.5 to "Power rule and its consequences."
14. Exercise 2.2.2 should just ask for the power rule, the second item is asked for in Exercise 2.2.3.
15. In Exercise 2.2.11, just use the identically equal sign to make it easier to read.
16. Add a note about what Proposition 2.2.9 means in view of Exercise 1.1.7 after the proof of the proposition.
17. Flip the two paragraphs about mapping properties and the n-to-1 behavior as it makes more sense in this order. This flips the figures 2.2 and 2.3.
18. Proper proof of the Cauchy-Hadamard theorem, it seemed to imply divergence about the series of the absolute values.
19. Add the word "absolutely" to Figure 2.4.
20. Reword Remark 2.3.5. We do definitely need addition, it is multiplication can could plausibly be put off for later.
21. In the proof of 2.4.6, save the "the series converges" only after we proved what it is.
22. Emphasize the idea of factoring out the zero out of a power series after the proof of the identity theorem (2.4.7).
23. In Exercise 2.4.17 emphasize to only show uniqueness.
24. On page 54 when comparing the definitions of line integrals, fix the right hand side and also add an underbrace that shows that the right hand side is precisely our previous definition. Also write $$\gamma(t) = x(t) + i\, y(t)$$ instead of the notation as a point in the plane.
25. After the definition of the line integral, note that the definition still holds even if the derivative would be zero.
26. In simple justification of reparametrization, use $$h' > 0$$ and $$h' < 0$$ instead of increasing decreasing to avoid zero derivative.
27. In Exercise 3.1.5, say $$f\equiv 1$$ instead of $$f=1$$ for emphasis.
28. Fix Proposition 3.1.7 to mention that the new path might not be a path if we are disallowing zero derivatives.
29. In the remark after Definition 3.2.5 emphasize "is" and "is equivalent to" to make it clear what is the difference that we are talking about.
30. In proof of 3.2.9 make sure to state that $$\alpha$$ is a complex number.
31. In proof of Theorem 3.3.11, no need to handle the case when $$P(z)$$ is constant, it is nonconstant by definition.
32. After the proof of 3.3.8, in the example, make it centered at $$p$$ to fix an erratum and mention again that the sup norm is $$M.$$
33. In proof A of 3.4.1, mention that we are applying Cauchy-Goursat (though it could also be one of the other Cauchy theorems).
34. Reword Definition 3.4.4 to read more naturally hopefully.
35. In proof of 3.4.5, one of the equalities was written as an inequality so fix that (it was not really wrong).
36. On page 91, checking that the formula for the principal branch of the logarithm works, note that $$L(1)=0=\operatorname{Log}(1)$$ to start with.
37. On page 95, at the end of the proof of Proposition 4.1.2, it says that $$L_j$$ are branches of $$\log$$ but they are branches of $$\log (z-p).$$
38. In Definition 4.2.1, use "open" rather than "domain", connected is not needed.
39. Improve the wording of proof of Theorem 4.2.2, it was misleading in a couple of bits.
40. Emphasize before Proposition 4.3.7 that simply-connectedness is a topological concept.
41. After statement of Theorem 4.4.2 (Laurent series), expand a bit on the convergence of such a series using what we know about power series.
42. In the introduction to subsection 4.5.1, emphasize that by "path" in this section we mean continuous. Also drop the footnote on definition 4.5.1, I think it was more misleading than helpful.
43. In Proposition 4.5.6 drop the "piecewise-$$C^1$$" it is not really needed any paths will do, though we only really need it for piecewise-$$C^1 .$$
44. Replace the quote at the beginning of chapter 5 with one that irritates me less given what's happening in the world.
45. Tighten up section 5.1 a bit.
46. Add Exercise 5.1.7 to prove L'Hopital's rule.
47. In the paragraph after definition 5.2.1, give parenthetical example of a pole and an essential singularity.
48. In the proof of Theorem 5.2.2, say $$h$$ is holomorphic on $$U$$ rather than just "near $$p$$."
49. Exercise 5.2.8 was worded a bit vaguely, the function is supposed to be "not identically zero".
50. In Corollary 5.2.3, in (i) make the conclusion that $$g$$ has a removable singularity and in (ii) make the hypothesis that it is bounded. that is the way it is meant to go.
51. Exercise 5.2.12, the $$f$$ should be "not identically zero."
52. In the exercises in section 5.3, emphasize that residue theorem should be used for the computations (we're not interested in exercising other calculus tricks here).
53. Rewrite the proof of Rouché (5.4.2) to not replace $$g$$ with $$-g$$ which I forgot about (erratum). The inequality is slightly less appealing, but on the other hand you use the principal branch of log.
54. In Rouché, the winding number should be 0 or 1 for all $$z \notin \Gamma,$$ rather than $$z \in U .$$
55. In section 5.6, add a final note about possibly using inverse function theorem to get the holomorphicity. Also add another note about using the argument principle to prove Lemma 5.6.2.
56. Shortening/tightening some wording to improve the flow improves also the pagination a tiny bit in sections 6.2 and 6.3
57. In the proof of Montel (Theorem 6.2.2), it is slightly easier to just apply fundamental theorem of calculus more directly to the difference $$f(z)-f(p).$$
58. Add Exercise 6.2.10, which is a nice application of Montel to characterizing the radius of convergence.
59. Simplify the statement of Riemann mapping (Theorem 6.3.1). That is, just say both existence and uniqueness at once, instead of following what the proof does.
60. In the proof of Riemann mapping theorem, when showing $$|f'(p)| < |h'(p)|$$ we change $$g(p)$$ to $$\varphi_{-g(p)}(0)$$ and then back again, that is unnecessary. Just leave the $$2g(p)$$ be and change just $$g'(p).$$
61. Further on in the proof of RMT, it says that $$|f_n'(p)|$$ is an increasing sequence which is not right, though we could assume it is. Better to just note that $$0 < |f_n'(p)| \leq |f'(p)|$$ for any $$n.$$
62. Add Exercise 7.3.11 that requires infinitely many reflections.
63. On the bottom of the first page of 9.1, we expand on why the series for $$f$$ cannot converge on the square.
64. In the proof of the reflection principle (10.1.1), mention that $$F$$ is continuous. That is rather immediate, but it should be stated.
65. Before Definition 10.1.2, be even less formal about the introduction of "real-analytic curve" as it seems like we are giving the actual definition but, it's not actually equivalent to 10.1.2.
66. Before Corollary 10.1.3, remove the sentence about "switching sides" it is more confusing than helpful.
67. Corollary 10.1.4 is unnecessarily restrictive, no need for $$\partial\mathbb{D} \subset U$$ if we say $$f(\partial\mathbb{D} \cap U) \subset \partial \mathbb{D}$$ instead of $$f(\partial\mathbb{D}) \subset \partial \mathbb{D}.$$
68. Make the footnote in Definition 10.2.5 into a normal paragraph. The comment is needed a bit later in the monodromy theorem and so shouldn't be relegated to a footnote.
69. Be more precise in the statement of Proposition 10.2.11.
70. In Corollary 10.2.15, only assume that $$V$$ is simply connected. In the proof of that theorem, explicitly note that there is only one inverse as $$U$$ is connected.
71. Flip the order of proof in Proposition A.2.13 to be more logical.
72. In Exercise A.3.11, only ask for the part of the proposition that was not yet proved to avoid confusing matters.
73. Theorem A.5.7 should mention that $$X$$ ought not be empty.
74. A little bit of cleanup of the proof of Theorem B.3.16.
75. Clean up the proof of Proposition B.2.1 to not use sequences as it's simpler without.
76. Some minor clarifications and fixes throughout.
77. Fix the known errata.

December 18th 2020 edition (version 1.1):

Very minor update. Minor wording/grammar improvements. Fix the accents on Rouché and Poincaré.

September 10th 2020 edition (version 1.0):

First version.