So we can now take this expression and rewrite it under the condition of du is equal to zero.

我们利用这个公式,并在du=0的条件下将它重写。

麻省理工公开课 - 热力学与动力学课程节选

I differentiate a second time and check the sign, so the second order condition, I differentiate this expression again with respect to q1.

我们对它进行二次求导然后看符号,这个式子的二阶导数,就是一阶导数再对q1进行求导

耶鲁公开课 - 博弈论课程节选