<aside> 💡 一个装满二阶导的方阵

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• Hessian Matrix 是针对 $f: R^n \rightarrow R$ ，输入是一个向量，输出一个标量。
• 每一行都是先对 $x_i$ 求导，再对其他求二阶导。

$$ \mathbf{H}{f}=\left[\begin{array}{cccc}\frac{\partial^{2} f}{\partial x{1}^{2}} & \frac{\partial^{2} f}{\partial x_{1} \partial x_{2}} & \cdots & \frac{\partial^{2} f}{\partial x_{1} \partial x_{n}} \\\frac{\partial^{2} f}{\partial x_{2} \partial x_{1}} & \frac{\partial^{2} f}{\partial x_{2}^{2}} & \cdots & \frac{\partial^{2} f}{\partial x_{2} \partial x_{n}} \\\vdots & \vdots & \ddots & \vdots \\\frac{\partial^{2} f}{\partial x_{n} \partial x_{1}} & \frac{\partial^{2} f}{\partial x_{n} \partial x_{2}} & \cdots & \frac{\partial^{2} f}{\partial x_{n}^{2}}\end{array}\right] $$