diff --git a/pages/Math416/Exam_reviews/Math416_E1.md b/pages/Math416/Exam_reviews/Math416_E1.md index 3bcb4ae..21186ac 100644 --- a/pages/Math416/Exam_reviews/Math416_E1.md +++ b/pages/Math416/Exam_reviews/Math416_E1.md @@ -553,7 +553,18 @@ $$ ### Power function -### Inverse trigonometric functions +For any two complex numbers $a,b$, we can define the power function as + +$$ +a^b = e^{b\log a} +$$ + +> Example: +> +> $$i^i=e^{i\ln i}=e^{i(\ln 1+i\frac{\pi}{2})}=e^{-\frac{\pi}{2}} $$ +> +> $$e^{i\pi}=-1$$ + ## Chapter 5 Power Series @@ -573,13 +584,46 @@ $$ \sum_{n=0}^\infty z^n = \frac{1}{1-z}, \quad |z|<1 $$ - ### Radius/Region of convergence +The radius of convergence of a power series is the largest number $R$ such that the series converges for all $z$ with $|z-z_0| Example: +> +> Evaluate $$\int_{|z|=2} \frac{z}{z-1} dz$$ +> +> Note that if we let $f(\zeta)=\zeta$ and $z=1$ is inside the circle, then we can use Cauchy Integral Formula for circle $C_r$ to evaluate the integral. +> +> So we have +> +> $$\int_{|z|=2} \frac{z}{z-1} dz = 2\pi i f(1) = 2\pi i$$ + +General Cauchy Integral Formula for circle $C_r$: + +$$ +f^{(n)}(z)=\frac{n!}{2\pi i}\int_{C_r} \frac{f(\zeta)}{(\zeta-z)^{n+1}} d\zeta +$$ + +> Example: +> +> Evaluate $$\int_{C}\frac{\sin z}{z^{38}}dz$$ +> +> Note that if we let $f(\zeta)=\sin \zeta$ and $z=0$ is inside the circle, then we can use General Cauchy Integral Formula for circle $C_r$ to evaluate the integral. +> +> So we have +> +> $$\int_{C}\frac{\sin z}{z^{38}}dz = \frac{2\pi i}{37!} f^{(37)}(0) = \frac{2\pi i}{37!} \sin ^{(37)}(0)$$ +> +> Note that $\sin ^{(n)}(0)=\begin{cases} 0,& n\equiv 0 \pmod{4}\\ +1,& n\equiv 1 \pmod{4}\\ +0,& n\equiv 2 \pmod{4}\\ +-1,& n\equiv 3 \pmod{4} +\end{cases}$ +> +> So we have +> +> $$\int_{C}\frac{\sin z}{z^{38}}dz = \frac{2\pi i}{37!} \sin ^{(37)}(0) = \frac{2\pi i}{37!} \cdot 1 = \frac{2\pi i}{37!}$$ + +_Cauchy integral is a easier way to evaluate the integral._ + +### Liouville's Theorem + +If a function $f$ is entire (holomorphic on $\mathbb{C}$) and bounded, then $f$ is constant. + +### Finding power series of holomorphic functions + +If $f$ is holomorphic on a disk $|z-z_0| Example: +> +> If $q(z)=(z-1)(z-2)(z-3)$, find the power series of $q(z)$ centered at $z=0$. +> +> Note that $q(z)$ is holomorphic on $\mathbb{C}$ and $q(z)=0$ at $z=1,2,3$. +> +> So we can use the power series of $q(z)$ centered at $z=1$. +> +> To solve this, we can simply expand $q(z)=(z-1)(z-2)(z-3)$ and get $q(z)=z^3-6z^2+11z-6$. +> +> So we have $a_0=q(1)=-6$, $a_1=q'(1)=3z^2-12z+11=11$, $a_2=\frac{q''(1)}{2!}=\frac{6z-12}{2}=-3$, $a_3=\frac{q'''(1)}{3!}=\frac{6}{6}=1$. +> +> So the power series of $q(z)$ centered at $z=1$ is +> +> $$q(z)=-6+11(z-1)-3(z-1)^2+(z-1)^3$$ + +### Fundamental Theorem of Algebra + +Every non-constant polynomial with complex coefficients has a root in $\mathbb{C}$. + +Can be factored into linear factors: + +$$ +p(z)=a_n(z-z_1)(z-z_2)\cdots(z-z_n) +$$ + +We can treat holomorphic functions as polynomials. + +$f$ has zero of order $m$ at $z_0$ if and only if $f(z)=(z-z_0)^m g(z)$ for some holomorphic $g(z)$ and $g(z_0)\neq 0$. + +### Zeros of holomorphic functions + +If $f$ is holomorphic on a disk $|z-z_0| Example: +> +> Find zeros of $f(z)=\cos(z\frac{\pi}{2})$ +> +> Note that $f(z)=0$ if and only if $z\frac{\pi}{2}=(2k+1)\frac{\pi}{2}$ for some integer $k$. +> +> So the zeros of $f(z)$ are $z=(2k+1)$ for some integer $k$. +> +> The order of the zero is $1$ since $f'(z)=-\frac{\pi}{2}\sin(z\frac{\pi}{2})$ and $f'(z)\neq 0$ for all $z=(2k+1)$. + +If $f$ vanishes to infinite order at $z_0$ (that is, $f(z_0)=f'(z_0)=f''(z_0)=\cdots=0$), then $f(z)\equiv 0$ on the connected open set $U$ containing $z_0$. + +### Identity Theorem + +If $f$ and $g$ are holomorphic on a connected open set $U\subset\mathbb{C}$ and $f(z)=g(z)$ for all $z$ in a subset of $U$ that has a limit point in $U$, then $f(z)=g(z)$ for all $z\in U$. + +Key: consider $h(z)=f(z)-g(z)$, prove $h(z)\equiv 0$ on $U$ by applying the zero of holomorphic function. + +### Weierstrass Theorem + +Limit of a sequence of holomorphic functions is holomorphic. + +Let $f_n$ be a sequence of holomorphic functions on a domain $D\subset\mathbb{C}$ that converges uniformly to $f$ on every compact subset of $D$. Then $f$ is holomorphic on $D$. + +### Maximum Modulus Principle + +If $f$ is a non-constant holomorphic function on a domain $D\subset\mathbb{C}$, then $|f|$ does not attain a maximum value in $D$. + +#### Corollary: Minimum Modulus Principle + +If $f$ is a non-constant holomorphic function on a domain $D\subset\mathbb{C}$, then $\frac{1}{f}$ does not attain a minimum value in $D$. + +### Schwarz Lemma + +If $f$ is a holomorphic function on the unit disk $|z|<1$ and $|f(z)|\leq |z|$, then $|f'(0)|\leq 1$. + diff --git a/pages/Math416/Exam_reviews/Math416_Final.md b/pages/Math416/Exam_reviews/Math416_Final.md new file mode 100644 index 0000000..fa3f5fc --- /dev/null +++ b/pages/Math416/Exam_reviews/Math416_Final.md @@ -0,0 +1,126 @@ +# Math 416 Final Review + +Story after Cauchy's theorem + +## Chapter 7: Cauchy's Theorem + +### Existence of harmonic conjugate + +Suppose $f=u+iv$ is holomorphic on a domain $U\subset\mathbb{C}$. Then $u=\Re f$ is harmonic on $U$. That is $\Delta u=\frac{\partial^2 u}{\partial x^2}+\frac{\partial^2 u}{\partial y^2}=0$. + +Moreover, there exists $g\in O(U)$ such that $g$ is unique up to an additive imaginary constant. + +> Example: +> +> Find a harmonic conjugate of $u(x,y)=\log|\frac{z}{z-1}|$ +> +> Note that $\log(\frac{z}{z-1})=\log \left|\frac{z}{z-1}\right|+i(\arg(z)-\arg(z-1))$ is harmonic on $\mathbb{C}\setminus\{1\}$. +> +> So the harmonic conjugate of $u(x,y)=\log|\frac{z}{z-1}|$ is $v(x,y)=\arg(z)-\arg(z-1)+C$ where $C$ is a constant. +> +> Note that the harmonic conjugate may exist locally but not globally. (e.g. $u(x,y)=\log|z(z-1)|$ has a local harmonic conjugate $i(\arg(z)+\arg(z-1)+C)$ but this is not a well defined function since $\arg(z)+\arg(z-1)$ is not single-valued.) + +### Corollary for harmonic functions + +#### Theorem 7.19 + +Harmonic function are infinitely differentiable. + +#### Theorem 7.20 + +Mean value property of harmonic functions. + +Let $u$ be harmonic on an open set of $\Omega$, then + +$$u(z_0)=\frac{1}{2\pi}\int_0^{2\pi} u(z_0+re^{i\theta}) d\theta$$ + +for any $z_0\in\Omega$ and $r>0$ such that $D(z_0,r)\subset\Omega$. + +#### Theorem 7.21 + +Identity theorem for harmonic functions. + +Let $u$ be harmonic on a domain $\Omega$. If $u=0$ on some open set $G\subset\Omega$, then $u\equiv 0$ on $\Omega$. + +#### Theorem 7.22 + +Maximum principle for harmonic functions. + +Let $u$ be a non-constant real-valued harmonic function on a domain $\Omega$. Then $|u|$ does not attain a maximum value in $\Omega$. + +## Chapter 8: Laurent Series and Isolated Singularities + +### Laurent Series + +Laurent series is a generalization of Taylor series. + +Laurent series is a power series of the form + +$$f(z)=\sum_{n=-\infty}^{\infty} a_n (z-z_0)^n$$ + +where + +$$ +a_k=\frac{1}{2\pi i}\int_{C_r}\frac{f(z)}{(z-z_0)^{k+1}}dz +$$ + +The series converges on an annulus $R_1<|z-z_0| Example: +> +> $f(z)=\frac{e^z-1}{z^2}$ has a removable singularity at $z=0$. +> +> The Laurent series of $f$ at $z=0$ can be found using the Taylor series of $e^z-1$ at $z=0$. +> +> $$e^z-1=z+\frac{z^2}{2!}+\frac{z^3}{3!}+\cdots$$ +> +> So the Laurent series of $f$ at $z=0$ is +> +> $$f(z)=\frac{1}{z^2}+\frac{1}{z}+\sum_{n=0}^{\infty}\frac{z^n}{n!}$$ +> +> The principle part is zero, so $z=0$ is a removable singularity. + +#### Poles + + + + + + + + + + + + + + + + + + + + +