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Math 4023 Complex Analysis L1 Course Outline
Math 2033, Math 2043 or Math 3033 This course is for learning complex analysis, which is a useful
part of mathematics that deals with problems involving complex numbers. It is a course that prepares
students to learn more about the powerful concepts and facts that can allow us to solve problems more cleverly.
Assessment Scheme Homeworks (30%), Midterm (30%), Final Exam (40%). All records of marks or grades
will be put on Canvas. Student Learning Resources Lecture Notes and Presentation Projects: Lecture notes
(and/or presentation projects) may be downloaded from Canvas. Intended Learning Outcomes
The School of Science Intended Learning Outcomes and the Math Department Intended Learning
Out- comes will be discribed in the first class. Upon the end of the course, you should have opportunities to and
should be able to ILO A: recognize the power of abstraction and generalization, and apply logical reasoning to
investigate mathematical work with independent judgement (cf Science ILO 1, Math ILO 3P) ILO B: communicate
effectively about math to peer and teaching staffs using available equipments or presentation softwares (cf Science ILO 4,
Math ILO 4) ILO C: apply rigorous deductive reasoning to analyze and solve problems related to math profession
(cf Science ILO 6, Math ILO 2) Teaching Approach There will be lectures by the instructor on materials presented in
the lecture notes with emphasis on proofs. By understanding proofs and doing homeworks, you will gradually attain
the ability for ILO A. Also, there will be tutorials run by the teaching assistants. In these tutorials, the teaching
assistances may do more examples or assess your written works or oral presentations. For possible presentation project,
you may do it via ZOOM. These will train you for ILO B. For ILO C, there will be written assignments and tests. Tentative Course Schedule (2 hours) Syllabus. Complex Sequences and Series (3 hours) Set Descriptions and Terminologies (3 hours) Continuity and Uniform Convergence (3 hours) Stereographic Projection (3 hours) Power Series. Cauchy-Riemann Equations (4 hours) Conformal Mappings, Contour Integrals, Cauchy Theory (4 hours) Harmonic Functions and Conjugates (4 hours) Morera’s Theorem, Isolated Singularities (4 hours) Residue Theory, Infinite Products. Lecture Notes for Math 4023 (Complex Analysis) c© Department of Mathematics, HKUST 1. Complex Sequences and Series Let N,Z,R denote the set of all positive integers, integers, real numbers respectively. For (x, y) ∈ R2, we will also write it as x + iy, where i = (0, 1) is to be defined as square root of −1. The set of all complex numbers is C = {x+ iy:x, y ∈ R} with addition, subtraction, multiplication and division defined as follows. For z = (x, y) = x+ iy, the real part, imaginary part, conjugate and absolute value of z are Re z = x, Im z = y, z = x− iy and |z| = √ x2 + y2 respectively. For z = x+ iy and w = u+ iv, define z + w = (x+ u) + i(y + v), z − w = (x− u) + i(y − v), zw = (xu− yv) + i(xv + yu). Note i2 = (0 + i)(0 + i) = −1 and ww = u2 + v2 = |w|2. So for w 6= 0, we may define z/w = zw/|w|2. Next, the distance between z and w is given by |z − w|. For z = x+ iy 6= 0, the argument of z, denoted by arg z, is the angle from the positive x-axis to the ray from (0, 0) to (x, y) in radian (modulo 2pi). For example, arg i is any of the numbers pi/2+2npi, where n ∈ Z . The expressions cis θ or eiθ denote cos θ+ i sin θ. Every nonzero complex number z has a polar representation z = r cis θ, where r = |z|, θ = arg z. Let z, w be complex numbers and n be an integer. The following properties can easily be verified: (1) z + z = 2 Re z, z − z = 2i Im z, zz = |z|2, z/z = cis(2 arg z) for z 6= 0, (2) z + w = z + w, z − w = z − w, zw = z w, z/w = z/w for w 6= 0, (3) |z +w| ≤ |z|+ |w|, |z − w| ≥ ∣∣|z| − |w|∣∣, |zw| = |z||w|, |z/w| = |z|/|w| for w 6= 0, (4) for z, w 6= 0, arg(zw) = arg z + argw, arg( z w ) = arg z − argw, arg(zn) = n arg z. In analysis, reasoning involving limits are very common and important. We will begin with the concept of the limit of a sequence. (For convenience, we will abbreviate “if and only if” by “iff” or “ ⇐⇒ ” in the sequel.) Definition.A sequence {z1, z2, z3, . . .} (or in short, {zn}) converges to z ∈ C (denoted by zn → z) iff for each ε > 0, there exists Nε ∈ N such that n ≥ Nε implies |zn − z| ≤ ε (in short, lim n→∞ |zn − z| = 0.) Otherwise, the sequence is said to diverge. Examples. (1) zn = zn converges to 0 if |z| < 1,converges to 1 if z = 1,diverges otherwise. (For |z| < 1, |zn−0| = |z|n→ 0 as n→∞. For z = 1, it is clear. For |z| ≥ 1 and z 6= 1, assume zn → w. Let ε = |z − 1|/2 > 0. Then there exists N ∈ N such that n ≥ N implies |zn−w| < ε. Now 2ε = |z− 1| ≤ |z|n|z− 1| = |zn+1− zn| ≤ |zn+1−w|+ |w− zn| < 2ε, which is a contradiction.) (2) lim n→∞ n n + z = 1, since ∣∣∣∣ nn+ z − 1 ∣∣∣∣ = |z||n+ z| = √ x2 + y2√ (n + x)2 + y2 → 0 as n→∞. Often the limit of a sequence is difficult or impossible to find. We now introduce a criterion that allows us to conclude a sequence is convergent without having to identify the limit explicitly. From math analysis course, we have the following 1 Definition. A sequence {zn} in C is a Cauchy sequence iff for each ε > 0, there exists Nε ∈ N such that m,n ≥ Nε implies |zm − zn| ≤ ε. Theorem. A sequence {zn} converges in C iff {Re zn}, {Imzn} converge in R iff {zn} is a Cauchy sequence. Proof. The case zn’s are real is proved in math analysis course. For the case zn = xn + iyn’s are complex, suppose {zn} converges to z = x + iy. Since |xn − x|, |yn − y| ≤ √|xn − x|2 + |yn − y|2 = |zn − z|, by the sandwich theorem, {xn}, {yn} converge to x, y respectively. Next suppose {xn}, {yn} converge (hence are Cauchy sequences). As |zn−zm|= √|xn − xm|2 + |yn − ym|2 ≤ |xn − xm|+ |yn − ym|, it follows {zn} is a Cauchy sequence. Finally, suppose {zn} is a Cauchy sequence. Since |xn − xm|, |yn − ym| ≤ |zn − zm|, both {xn}, {yn} are Cauchy sequences. Then {xn}, {yn} converge to some x, y respectively. Since |(xn + iyn) − (x + iy)| ≤ |xn − x|+ |yn − y|, by the sandwich theorem, {zn} converges to x+ iy. QED Definitions.A series ∞∑ k=1 zk = z1 + z2 + z3 + . . . converges iff the sequence z1, z1+z2, z1+z2+z3, . . . converges (iff Sn = z1 + z2 + . . .+ zn is a Cauchy sequence). Otherwise, the series is said to diverge. There are two simple tests for checking convergence of series, namely the term test and the absolute convergence test. The former provides a necessary condition for convergence and the latter provides a sufficient condition for convergence. Term Test. If ∞∑ k=1 zk converges, then lim n→∞ zn = limn→∞(Sn − Sn−1) = limn→∞Sn − limn→∞Sn−1 = 0. Absolute Convergence Test. If ∞∑ k=1 |zk| converges, then ∞∑ k=1 zk converges (because Tn = |z1|+|z2|+. . .+|zn| is a Cauchy sequence and for m > n, |Sm − Sn| = |zn+1 + zn+2 + . . .+ zm| ≤ |zn+1|+ |zn+2|+ . . .+ |zm| = Tm − Tn = |Tm − Tn|, forcing Sn to be a Cauchy sequence.) Examples. (1) ∞∑ k=1 1 k2 + i converges because ∞∑ k=1 ∣∣∣∣ 1k2 + i ∣∣∣∣ = ∞∑ k=1 1√ k4 + 1 ( ≤ ∞∑ k=1 1 k2 ) converges. (2) ∞∑ k=1 1 k + i diverges because Re ( ∞∑ k=1 1 k + i ) = Re ( ∞∑ k=1 k − i k2 + 1 ) = ∞∑ k=1 k k2 + 1 ( ≥ ∞∑ k=1 1 2k ) diverges. Exercises 1. For what complex values z will the following series converge (a) ∞∑ n=0 ( z 1 + z )n ; (b) ∞∑ n=1 nzn; (c) ∞∑ n=0 zn 1 + z2n ? 2. When will equality occur in the triangle inequality |w+ z| ≤ |w|+ |z|? That is, under what conditions on w and z will |w + z| = |w|+ |z|? 2 3. Establish the identity ∣∣∣∣∣ n∑ k=1 αkβk ∣∣∣∣∣ 2 = n∑ k=1 |αk|2 n∑ k=1 |βk|2 − ∑ 1≤k