我们说一个函数是可微的，如果这些偏导数存在。
And, we say that a function is differentiable if these things exist.
偏微分方程就是，跟函数各个偏导数有关的方程。
A partial differential equation is an equation that involves the partial derivatives of a function.
偏微分方程就是，跟函数各个偏导数有关的方程。
A partial differential equation is an equation that involves the partial derivatives of a function.
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