Let $U \subset \mathbb{R}^2$ be open. A function $u: U \to \mathbb{R}$ is harmonic if it is twice continuously\differentiable and if\$\Delta u = \frac{\partial^2 u}{\partial x^2} + \frac{\partial^2 u}{\partial y^2}$\is constant equal to 0.\1. Let $f = u + iv: U \to \mathbb{C}$ be holomorphic. Show that both $u: U \to \mathbb{R}$ and\$v: U \to \mathbb{R}$ are harmonic.\Hint: Use the Cauchy-Riemann equations.\2. Let $u: D_1(0) \to \mathbb{R}$ be harmonic. Show that there exists a $v: D_1(0) \to \mathbb{R}$ such\that $f = u + iv: D_1(0) \to \mathbb{C}$ is holomorphic.\Hint: Show that $g = \frac{\partial u}{\partial x} - i\frac{\partial u}{\partial y}$ is holomorphic in $D_1(0)$. Show that $g$ has a\primitive of the form $f = u + iv$.\Remark: Such a $v$ is called a harmonic conjugate of $u$ and it is uniquely\determined up to the choice of an additive constant.\3. Conclude that a harmonic function $u: D_1(0) \to \mathbb{R}$ is smooth, i.e. arbitrarily\often differentiable.
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### Part 1: Showing u and v are harmonic if f = u + iv is holomorphic ** Show moreβ¦
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Let f(z) = f(x + iy) = u(x,y) + iv(x,y), where u and v are twice continuously differentiable functions. (a) Show that if v is the harmonic conjugate of u then -u is the harmonic conjugate of v. (b) Let f(x + iy) = u(x,y) + iv(x,y) be analytic on a domain D. Assume that u and v do not vanish together at any point x + iy β D. Show that u/(u^2 + v^2) and -v/(u^2 + v^2) are harmonic functions. (c) Show that if f(z) is a complex valued harmonic function such that zf(z) is also harmonic, then f(z) is analytic.
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2. (a) Write the function f: β β β, f(z) = exp(Ε³) in the form f(z) = u(x,y) + iv(x,y) where u and v are real valued functions of the real variables x and y. (b) Is the function f: β β β, f(z) = exp(Ε³) (i) continuous at 0, (ii) differentiable at 0? Justify your answer in both cases. (c) Define what is understood for a function u: β" β β to be harmonic. (d) Show that the function u(x,y) = xeΛ£ cos y - yeΛ£ sin y is harmonic and find the most general holomorphic function, f, with real part u(x,y). Write f as a function of z.
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Harmonic Functions $\quad$ Suppose $u(x, y)$ and $v(x, y)$ have continuous second-order partial derivatives, $u_{x}=v_{y}$ and $u_{y}=-v_{x} .$ Show that $u$ and $v$ are harmonic functions.
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