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    "# 附录 D 变分法"
   ]
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   "metadata": {},
   "source": [
    "函数 $y(x)$ 可以看成一种操作符，即对于任意 $x$，返回一个输出 $y$。在这种情况下，我们可以定义泛函 $F[y]$：接受一个函数 $y(x)$，返回输出值 $F$。\n",
    "\n",
    "机器学习中一个常见的例子熵 $H[x]$，因为对于任何一个概率分布 $p(x)$，它返回了 $x$ 在该分布下的熵，因此 $p(x)$ 的熵也可以写作 $H[p]$。\n",
    "\n",
    "微积分中一个常见的问题是找到一个 $x$ 最大（小）化一个函数 $y(x)$，而变分法要解决的问题是找到一个 $y(x)$ 最大（小）化一个泛函 $F[y]$。\n",
    "\n",
    "例如变分法可以解释为什么两点之间直线最短，或者最大熵分布是高斯分布。\n",
    "\n",
    "传统的微积分给出：\n",
    "\n",
    "$$\n",
    "y(x+\\epsilon) = y(x) + \\frac{dy}{dx} \\epsilon + O(\\epsilon^2)\n",
    "$$\n",
    "\n",
    "以及\n",
    "\n",
    "$$\n",
    "y(x_1+\\epsilon_1, \\dots, x_D+\\epsilon_D) = y(x_1, \\dots, x_D) + \\sum_{i=1}^D \\frac{\\partial y}{\\partial x} \\epsilon_i + O(\\epsilon^2)\n",
    "$$\n",
    "\n",
    "类似的，泛函求导时，我们考虑一个 $y(x)$ 的小变化 $\\epsilon\\eta(x)$，（$\\eta(x)$ 是关于 $x$ 的任意函数），记泛函 $F[y]$ 对函数 $y(x)$ 的导数为 $\\frac{\\delta F[y]}{\\delta y(x)}$，定义：\n",
    "\n",
    "$$\n",
    "F[y(x)+\\epsilon\\eta(x)] = F[y(x)] + \\epsilon \\int \\frac{\\delta F[y]}{\\delta y(x)} \\eta(x) dx + O(\\epsilon^2)\n",
    "$$\n",
    "\n",
    "泛函达到驻点的条件为，对任意 $\\eta(x)$，有：\n",
    "\n",
    "$$\n",
    "\\int \\frac{\\delta F[y]}{\\delta y(x)} \\eta(x) dx = 0\n",
    "$$\n",
    "\n",
    "要对任意 $\\eta(x)$ 成立，$\\frac{\\delta F[y]}{\\delta y(x)}$ 必须为 0。\n",
    "\n",
    "考虑这样一个泛函：\n",
    "\n",
    "$$\n",
    "F[y] = \\int G(y(x), y'(x), x) dx\n",
    "$$\n",
    "\n",
    "其中，$y'(x)$ 是 $y(x)$ 关于 $x$ 的导数，**$y(x)$ 在积分区间的边界的值是固定的**。考虑 $y(x)$ 的变分，展开之后有：\n",
    "\n",
    "$$\n",
    "\\begin{align}\n",
    "F[y(x)+\\epsilon\\eta(x)] \n",
    "& = \\int G(y+\\epsilon\\eta, y'+\\epsilon\\eta', x) dx \\\\\n",
    "& = \\int G(y, y', x) dx + \\epsilon \\int \n",
    "\\left\\{ \\frac{\\partial G}{\\partial y} \\eta(x) + \n",
    "\\frac{\\partial G}{\\partial y'} \\eta'(x)\\right\\} dx + O(\\epsilon^2) \\\\\n",
    "& = F[y(x)] + \\epsilon \\int \n",
    "\\left\\{ \\frac{\\partial G}{\\partial y} \\eta(x) + \n",
    "\\frac{\\partial G}{\\partial y'} \\eta'(x)\\right\\} dx + O(\\epsilon^2) \\\\\n",
    "\\end{align}\n",
    "$$\n",
    "\n",
    "为了得到定义的形式，对积分的第二部分进行分部积分：\n",
    "\n",
    "$$\n",
    "\\int \\frac{\\partial G}{\\partial y'} \\eta'(x) dx = \\left.\\frac{\\partial G}{\\partial y'} \\eta(x) \\right|_x - \\int \\frac{d}{dx} \\left(\\frac{\\partial G}{\\partial y'}\\right) \\eta(x) dx\n",
    "$$\n",
    "\n",
    "考虑到 $y(x)$ 在边界上的值是固定的，因此在边界上 $\\eta(x)$ 必须为 0，于是我们有\n",
    "\n",
    "$$\n",
    "F[y(x)+\\epsilon\\eta(x)] \n",
    "= F[y(x)] + \\epsilon \\int \n",
    "\\left\\{ \\frac{\\partial G}{\\partial y} - \n",
    "\\frac{d}{dx} \\left(\\frac{\\partial G}{\\partial y'}\\right) \\right\\}\\eta(x) dx + O(\\epsilon^2) \\\\\n",
    "$$\n",
    "\n",
    "令变分为 0，我们有\n",
    "\n",
    "$$\n",
    "\\frac{\\partial G}{\\partial y} - \n",
    "\\frac{d}{dx} \\left(\\frac{\\partial G}{\\partial y'}\\right) = 0\n",
    "$$\n",
    "\n",
    "这就是著名的欧拉-拉格朗日方程（`Euler-Lagrange equation`）。\n",
    "\n",
    "例如 $G=y(x)^2+\\left(y'(x)\\right)^2$，欧拉-拉格朗日方程为：\n",
    "\n",
    "$$\n",
    "y(x) - \\frac{d^2y}{dx^2} = 0\n",
    "$$\n",
    "\n",
    "利用边界条件，我们可以解出这个方程。\n",
    "\n",
    "在很多问题中，如果我们的被积函数为 $G(y,x)$，那么驻点的要求为\n",
    "\n",
    "$$\n",
    "\\frac{\\partial G}{\\partial y(x)}= 0\n",
    "$$"
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