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| 200 | +<main> |
| 201 | +<h1 class="unnumbered" |
| 202 | +id="complex-analysis-general-exam-spring-2022">COMPLEX ANALYSIS GENERAL |
| 203 | +EXAM SPRING 2022</h1> |
| 204 | +<p>Solve as many problems as you can. Full solutions on a smaller number |
| 205 | +of problems will be worth more than partial solutions on several |
| 206 | +problems. Throughout |
| 207 | +<math role="math" aria-label="\mathbb{D}=\{z \in \mathbb{C}:|z|<1\}" display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>𝔻</mi><mo>=</mo><mo stretchy="false" form="prefix">{</mo><mi>z</mi><mo>∈</mo><mi>ℂ</mi><mo>:</mo><mo stretchy="false" form="prefix">|</mo><mi>z</mi><mo stretchy="false" form="prefix">|</mo><mo><</mo><mn>1</mn><mo stretchy="false" form="postfix">}</mo></mrow><annotation encoding="application/x-tex">\mathbb{D}=\{z \in \mathbb{C}:|z|<1\}</annotation></semantics></math>. |
| 208 | +Don’t use any of the Picard theorems.</p> |
| 209 | +<h2 class="unnumbered" id="problem-1">Problem 1</h2> |
| 210 | +<p>Compute, for |
| 211 | +<math role="math" aria-label="0<\alpha<1" display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mn>0</mn><mo><</mo><mi>α</mi><mo><</mo><mn>1</mn></mrow><annotation encoding="application/x-tex">0<\alpha<1</annotation></semantics></math></p> |
| 212 | +<p><math role="math" aria-label="\int_{0}^{\infty} \frac{1}{x^{\alpha}(1+x)} d x" display="block" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><msubsup><mo>∫</mo><mn>0</mn><mi>∞</mi></msubsup><mfrac><mn>1</mn><mrow><msup><mi>x</mi><mi>α</mi></msup><mo stretchy="false" form="prefix">(</mo><mn>1</mn><mo>+</mo><mi>x</mi><mo stretchy="false" form="postfix">)</mo></mrow></mfrac><mi>d</mi><mi>x</mi></mrow><annotation encoding="application/x-tex">\int_{0}^{\infty} \frac{1}{x^{\alpha}(1+x)} d x</annotation></semantics></math></p> |
| 213 | +<p>Show all estimates.</p> |
| 214 | +<h2 class="unnumbered" id="problem-2">Problem 2</h2> |
| 215 | +<p>Suppose that |
| 216 | +<math role="math" aria-label="f" display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mi>f</mi><annotation encoding="application/x-tex">f</annotation></semantics></math> |
| 217 | +is an entire function, and that there are constants |
| 218 | +<math role="math" aria-label="a, b>0" display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>a</mi><mo>,</mo><mi>b</mi><mo>></mo><mn>0</mn></mrow><annotation encoding="application/x-tex">a, b>0</annotation></semantics></math> |
| 219 | +so that</p> |
| 220 | +<p><math role="math" aria-label="|f(z)| \leq a+b|z| \text { for all } z \in \mathbb{C}" display="block" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mo stretchy="false" form="prefix">|</mo><mi>f</mi><mo stretchy="false" form="prefix">(</mo><mi>z</mi><mo stretchy="false" form="postfix">)</mo><mo stretchy="false" form="prefix">|</mo><mo>≤</mo><mi>a</mi><mo>+</mo><mi>b</mi><mo stretchy="false" form="prefix">|</mo><mi>z</mi><mo stretchy="false" form="prefix">|</mo><mrow><mspace width="0.333em"></mspace><mtext mathvariant="normal"> for all </mtext><mspace width="0.333em"></mspace></mrow><mi>z</mi><mo>∈</mo><mi>ℂ</mi></mrow><annotation encoding="application/x-tex">|f(z)| \leq a+b|z| \text { for all } z \in \mathbb{C}</annotation></semantics></math></p> |
| 221 | +<p>Show that |
| 222 | +<math role="math" aria-label="f" display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mi>f</mi><annotation encoding="application/x-tex">f</annotation></semantics></math> |
| 223 | +is a polynomial of degree at most one.</p> |
| 224 | +<h2 class="unnumbered" id="problem-3">Problem 3</h2> |
| 225 | +<p>Give an explicit example of an unbounded harmonic function |
| 226 | +<math role="math" aria-label="u: \mathbb{D} \rightarrow(0,+\infty)" display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>u</mi><mo>:</mo><mi>𝔻</mi><mo>→</mo><mo stretchy="false" form="prefix">(</mo><mn>0</mn><mo>,</mo><mi>+</mi><mi>∞</mi><mo stretchy="false" form="postfix">)</mo></mrow><annotation encoding="application/x-tex">u: \mathbb{D} \rightarrow(0,+\infty)</annotation></semantics></math> |
| 227 | +with the property that</p> |
| 228 | +<p><math role="math" aria-label="\lim _{z \rightarrow \zeta} u(z)=0" display="block" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><munder><mi mathvariant="normal">lim</mi><mrow><mi>z</mi><mo>→</mo><mi>ζ</mi></mrow></munder><mi>u</mi><mo stretchy="false" form="prefix">(</mo><mi>z</mi><mo stretchy="false" form="postfix">)</mo><mo>=</mo><mn>0</mn></mrow><annotation encoding="application/x-tex">\lim _{z \rightarrow \zeta} u(z)=0</annotation></semantics></math></p> |
| 229 | +<p>for all |
| 230 | +<math role="math" aria-label="\zeta \in \partial \mathbb{D}" display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>ζ</mi><mo>∈</mo><mi>∂</mi><mi>𝔻</mi></mrow><annotation encoding="application/x-tex">\zeta \in \partial \mathbb{D}</annotation></semantics></math> |
| 231 | +with |
| 232 | +<math role="math" aria-label="\zeta \neq 1" display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>ζ</mi><mo>≠</mo><mn>1</mn></mrow><annotation encoding="application/x-tex">\zeta \neq 1</annotation></semantics></math>. |
| 233 | +It is acceptable to leave your answer as the real (or imaginary) part of |
| 234 | +an explicit holomorphic function.</p> |
| 235 | +<h2 class="unnumbered" id="problem-4">Problem 4</h2> |
| 236 | +<p>Let |
| 237 | +<math role="math" aria-label="S=\{z \in \mathbb{C}: 0<\operatorname{Re}(z)<1\}" display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>S</mi><mo>=</mo><mo stretchy="false" form="prefix">{</mo><mi>z</mi><mo>∈</mo><mi>ℂ</mi><mo>:</mo><mn>0</mn><mo><</mo><mrow><mi mathvariant="normal">Re</mi><mo>⁡</mo></mrow><mo stretchy="false" form="prefix">(</mo><mi>z</mi><mo stretchy="false" form="postfix">)</mo><mo><</mo><mn>1</mn><mo stretchy="false" form="postfix">}</mo></mrow><annotation encoding="application/x-tex">S=\{z \in \mathbb{C}: 0<\operatorname{Re}(z)<1\}</annotation></semantics></math>. |
| 238 | +Suppose that |
| 239 | +<math role="math" aria-label="f: \bar{S} \rightarrow \mathbb{C}" display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>f</mi><mo>:</mo><mover><mi>S</mi><mo accent="true">‾</mo></mover><mo>→</mo><mi>ℂ</mi></mrow><annotation encoding="application/x-tex">f: \bar{S} \rightarrow \mathbb{C}</annotation></semantics></math> |
| 240 | +is bounded, continuous, and that |
| 241 | +<math role="math" aria-label="\left.f\right|_{S}" display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><msub><mrow><mi>f</mi><mo stretchy="true" form="postfix">|</mo></mrow><mi>S</mi></msub><annotation encoding="application/x-tex">\left.f\right|_{S}</annotation></semantics></math> |
| 242 | +is analytic. If</p> |
| 243 | +<p><math role="math" aria-label="\sup _{t \in \mathbb{R}} \max (|f(i t)|,|f(1+i t)|) \leq 1," display="block" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><munder><mi mathvariant="normal">sup</mi><mrow><mi>t</mi><mo>∈</mo><mi>ℝ</mi></mrow></munder><mrow><mi mathvariant="normal">max</mi><mo>⁡</mo></mrow><mo stretchy="false" form="prefix">(</mo><mo stretchy="false" form="prefix">|</mo><mi>f</mi><mo stretchy="false" form="prefix">(</mo><mi>i</mi><mi>t</mi><mo stretchy="false" form="postfix">)</mo><mo stretchy="false" form="prefix">|</mo><mo>,</mo><mo stretchy="false" form="prefix">|</mo><mi>f</mi><mo stretchy="false" form="prefix">(</mo><mn>1</mn><mo>+</mo><mi>i</mi><mi>t</mi><mo stretchy="false" form="postfix">)</mo><mo stretchy="false" form="prefix">|</mo><mo stretchy="false" form="postfix">)</mo><mo>≤</mo><mn>1</mn><mo>,</mo></mrow><annotation encoding="application/x-tex">\sup _{t \in \mathbb{R}} \max (|f(i t)|,|f(1+i t)|) \leq 1,</annotation></semantics></math></p> |
| 244 | +<p>show that |
| 245 | +<math role="math" aria-label="|f(z)| \leq 1" display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mo stretchy="false" form="prefix">|</mo><mi>f</mi><mo stretchy="false" form="prefix">(</mo><mi>z</mi><mo stretchy="false" form="postfix">)</mo><mo stretchy="false" form="prefix">|</mo><mo>≤</mo><mn>1</mn></mrow><annotation encoding="application/x-tex">|f(z)| \leq 1</annotation></semantics></math> |
| 246 | +for all |
| 247 | +<math role="math" aria-label="z \in S" display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>z</mi><mo>∈</mo><mi>S</mi></mrow><annotation encoding="application/x-tex">z \in S</annotation></semantics></math>.<br /> |
| 248 | +Suggestion: consider, for |
| 249 | +<math role="math" aria-label="\varepsilon>0" display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>ε</mi><mo>></mo><mn>0</mn></mrow><annotation encoding="application/x-tex">\varepsilon>0</annotation></semantics></math>, |
| 250 | +the function |
| 251 | +<math role="math" aria-label="f_{\varepsilon}(z)=\frac{f(z)}{1+\varepsilon z}" display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><msub><mi>f</mi><mi>ε</mi></msub><mo stretchy="false" form="prefix">(</mo><mi>z</mi><mo stretchy="false" form="postfix">)</mo><mo>=</mo><mfrac><mrow><mi>f</mi><mo stretchy="false" form="prefix">(</mo><mi>z</mi><mo stretchy="false" form="postfix">)</mo></mrow><mrow><mn>1</mn><mo>+</mo><mi>ε</mi><mi>z</mi></mrow></mfrac></mrow><annotation encoding="application/x-tex">f_{\varepsilon}(z)=\frac{f(z)}{1+\varepsilon z}</annotation></semantics></math>. |
| 252 | +Show that |
| 253 | +<math role="math" aria-label="\left|f_{\varepsilon}\right| \leq 1" display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mrow><mo stretchy="true" form="prefix">|</mo><msub><mi>f</mi><mi>ε</mi></msub><mo stretchy="true" form="postfix">|</mo></mrow><mo>≤</mo><mn>1</mn></mrow><annotation encoding="application/x-tex">\left|f_{\varepsilon}\right| \leq 1</annotation></semantics></math> |
| 254 | +for all |
| 255 | +<math role="math" aria-label="\varepsilon>0" display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>ε</mi><mo>></mo><mn>0</mn></mrow><annotation encoding="application/x-tex">\varepsilon>0</annotation></semantics></math>, |
| 256 | +and use this to conclude that |
| 257 | +<math role="math" aria-label="|f| \leq 1" display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mo stretchy="false" form="prefix">|</mo><mi>f</mi><mo stretchy="false" form="prefix">|</mo><mo>≤</mo><mn>1</mn></mrow><annotation encoding="application/x-tex">|f| \leq 1</annotation></semantics></math>.</p> |
| 258 | +<h2 class="unnumbered" id="problem-5">Problem 5</h2> |
| 259 | +<p>Let |
| 260 | +<math role="math" aria-label="\left(M_{n}\right)_{n=0}^{\infty}" display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><msubsup><mrow><mo stretchy="true" form="prefix">(</mo><msub><mi>M</mi><mi>n</mi></msub><mo stretchy="true" form="postfix">)</mo></mrow><mrow><mi>n</mi><mo>=</mo><mn>0</mn></mrow><mi>∞</mi></msubsup><annotation encoding="application/x-tex">\left(M_{n}\right)_{n=0}^{\infty}</annotation></semantics></math> |
| 261 | +be a sequence of positive real numbers. Assume that the series |
| 262 | +<math role="math" aria-label="\sum_{n=0}^{\infty} M_{n} z^{n}" display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><msubsup><mo>∑</mo><mrow><mi>n</mi><mo>=</mo><mn>0</mn></mrow><mi>∞</mi></msubsup><msub><mi>M</mi><mi>n</mi></msub><msup><mi>z</mi><mi>n</mi></msup></mrow><annotation encoding="application/x-tex">\sum_{n=0}^{\infty} M_{n} z^{n}</annotation></semantics></math> |
| 263 | +has radius of convergence at least 1 . Let |
| 264 | +<math role="math" aria-label="\mathcal{F}" display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mi>ℱ</mi><annotation encoding="application/x-tex">\mathcal{F}</annotation></semantics></math> |
| 265 | +be the set of holomorphic functions on |
| 266 | +<math role="math" aria-label="\mathbb{D}" display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mi>𝔻</mi><annotation encoding="application/x-tex">\mathbb{D}</annotation></semantics></math> |
| 267 | +which satisfy</p> |
| 268 | +<p><math role="math" aria-label="\left|\frac{f^{(n)}(0)}{n!}\right| \leq M_{n} \text { for all } n \in \mathbb{N} \cup\{0\} ." display="block" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mrow><mo stretchy="true" form="prefix">|</mo><mfrac><mrow><msup><mi>f</mi><mrow><mo stretchy="false" form="prefix">(</mo><mi>n</mi><mo stretchy="false" form="postfix">)</mo></mrow></msup><mo stretchy="false" form="prefix">(</mo><mn>0</mn><mo stretchy="false" form="postfix">)</mo></mrow><mrow><mi>n</mi><mi>!</mi></mrow></mfrac><mo stretchy="true" form="postfix">|</mo></mrow><mo>≤</mo><msub><mi>M</mi><mi>n</mi></msub><mrow><mspace width="0.333em"></mspace><mtext mathvariant="normal"> for all </mtext><mspace width="0.333em"></mspace></mrow><mi>n</mi><mo>∈</mo><mi>ℕ</mi><mo>∪</mo><mo stretchy="false" form="prefix">{</mo><mn>0</mn><mo stretchy="false" form="postfix">}</mo><mi>.</mi></mrow><annotation encoding="application/x-tex">\left|\frac{f^{(n)}(0)}{n!}\right| \leq M_{n} \text { for all } n \in \mathbb{N} \cup\{0\} .</annotation></semantics></math></p> |
| 269 | +<p>Show that |
| 270 | +<math role="math" aria-label="\mathcal{F}" display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mi>ℱ</mi><annotation encoding="application/x-tex">\mathcal{F}</annotation></semantics></math> |
| 271 | +is normal (i.e. the closure of |
| 272 | +<math role="math" aria-label="\mathcal{F}" display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mi>ℱ</mi><annotation encoding="application/x-tex">\mathcal{F}</annotation></semantics></math> |
| 273 | +is compact for the topology of uniform convergence on compact subsets of |
| 274 | +<math role="math" aria-label="\mathbb{D}" display="inline" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mi>𝔻</mi><annotation encoding="application/x-tex">\mathbb{D}</annotation></semantics></math> |
| 275 | +).</p> |
| 276 | +<p>? =0 <a href="#fn1" class="footnote-ref" id="fnref1" |
| 277 | +role="doc-noteref"><sup>1</sup></a></p> |
| 278 | +<p>??=0 <a href="#fn2" class="footnote-ref" id="fnref2" |
| 279 | +role="doc-noteref"><sup>2</sup></a></p> |
| 280 | +<section id="footnotes" class="footnotes footnotes-end-of-document" |
| 281 | +role="doc-endnotes"> |
| 282 | +<hr /> |
| 283 | +<ol> |
| 284 | +<li id="fn1"><p>Date: January 12, 2022.<a href="#fnref1" |
| 285 | +class="footnote-back" role="doc-backlink">↩︎</a></p></li> |
| 286 | +<li id="fn2"><p>Date: January 12, 2022.<a href="#fnref2" |
| 287 | +class="footnote-back" role="doc-backlink">↩︎</a></p></li> |
| 288 | +</ol> |
| 289 | +</section> |
| 290 | +</main> |
| 291 | +</body> |
| 292 | +</html> |
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