diff --git a/Exercise sheet 8/exercise_sheet_08.ipynb b/Exercise sheet 8/exercise_sheet_08.ipynb index 2740717..b69cf70 100644 --- a/Exercise sheet 8/exercise_sheet_08.ipynb +++ b/Exercise sheet 8/exercise_sheet_08.ipynb @@ -421,7 +421,38 @@ } }, "source": [ - "YOUR ANSWER HERE" + "**To proof**\n", + "\n", + "$\\frac{1}{V}\\mathbb{E}[ \\rho_T(r) ] = \\mathbb{P}(d_T(X,Y) = r)$\n", + "\n", + "**Proof**\n", + "\n", + "$$\n", + "\\frac{1}{V} \\mathbb{E}\\left[ \\rho_T(r)\\right]\n", + " = \\frac{1}{V} \\mathbb{E} \\left[\\frac{1}{V} \\sum_{x=0}^{V-1}\\sum_{y=0}^{V-1} \\mathbf{1}_{\\{d_T(x,y)=r\\}} \\right]\n", + " = \\frac{1}{V^2} \\mathbb{E} \\left[ \\sum_{x=0}^{V-1}\\sum_{y=0}^{V-1} \\mathbf{1}_{\\{d_T(x,y)=r\\}} \\right]\n", + " = \\frac{1}{V^2} \\sum_{x=0}^{V-1}\\sum_{y=0}^{V-1} \\mathbb{E} \\left[ \\mathbf{1}_{\\{d_T(x,y)=r\\}} \\right]\n", + "$$\n", + "\n", + "The order of summation is changed, as the sum of expectation values is equal to the expectation value of the sum.\n", + "The latter expectation value of the indicator function is exactly equal to the chance $\\mathbb{P}(d_T(x,y)=r)$ for given $x, y$.\n", + "For the uniformly distributed $X, Y$, we find $\\mathbb{P}(X = x) = \\frac{1}{V} = \\mathbb{P}(Y = y)$.\n", + "This allows us to write the right hand side as follows.\n", + "\n", + "$$\n", + "\\frac{1}{V} \\mathbb{E}\\left[ \\rho_T(r)\\right]\n", + " = \\frac{1}{V^2} \\sum_{x=0}^{V-1}\\sum_{y=0}^{V-1} \\mathbb{P}(d_T(x,y)=r)\n", + " = \\sum_{x=0}^{V-1}\\sum_{y=0}^{V-1} \\mathbb{P}(X = x) \\mathbb{P}(Y = y) \\mathbb{P}(d_T(x,y)=r)\n", + " = \\mathbb{P}(d_T(X,Y)=r),\n", + "$$\n", + "\n", + "which is what we sought.\n", + "\n", + "Using this result, it is just a matter of writing out the definition of an expectation value to get to the result.\n", + "\n", + "$$\n", + "\\mathbb{E}[d_T(X,Y)] = \\sum_{r=0}^\\infty r\\, \\mathbb{P}(d_T(X,Y) = r) = \\frac{1}{V}\\sum_{r=0}^\\infty r\\, \\mathbb{E}[ \\rho_T(r) ].\n", + "$$" ] }, { @@ -475,7 +506,36 @@ } }, "source": [ - "YOUR ANSWER HERE" + "**To proof**\n", + "\n", + "$$\n", + "\\mathbb{E}[d_T(X,Y)] \\approx c\\,V^{1/d_H}, \\qquad c = \\int_0^\\infty \\mathrm{d}x\\,x\\,f(x)\n", + "$$\n", + "\n", + "**Proof**\n", + "\n", + "$$\n", + "\\mathbb{E} \\left[ d_T(X,Y) \\right]\n", + " = \\frac{1}{V} \\sum_{r=0}^\\infty r\\, \\mathbb{E} \\left[ \\rho_T(r) \\right]\n", + " = \\frac{1}{V} \\sum_{r=0}^\\infty rV^{1-1/d_H}f(rV^{-1/d_H})\n", + " = \\frac{1}{V} \\sum_{r=0}^\\infty xV^{1/d_H} \\cdot V^{1-1/d_H}f(x)\n", + " = \\sum_{r=0}^\\infty xf(x),\n", + "$$\n", + "where the first equality sign is due to (2), the second due to the given assumption, the third using $x = rV^{-1/d_H}$.\n", + "\n", + "Now we approximate the summation by an integral.\n", + "\n", + "$$\n", + "\\sum_{r=0}^\\infty xf(x)\n", + " \\approx \\int_{r=0}^\\infty xf(x)dr\n", + " = V^{1/d_H} \\int_{x=0}^\\infty xf(x)dx\n", + " = cV^{1/d_H},\n", + "$$\n", + "using $\\frac{dr}{dx} = V^{1/d_H}$ for substitution.\n", + "This yields the desired approximation\n", + "$$\n", + " \\mathbb{E} \\left[ d_T(X,Y) \\right] \\approx cV^{1/d_H}.\n", + "$$" ] }, { @@ -602,6 +662,7 @@ " \n", " for idx_model, model in enumerate(models):\n", " # Calculate mean and standard deviation of the expectations.\n", + " # TODO: Look at whether I store the right data and do the right calculations.\n", " mu = np.mean(expectations[model], 1)\n", " sigma = np.std(expectations[model], 1)\n", "\n",