Every function whose derivative is linear is a quadratic function. 任意一个导数为线性函数的函数都是二次函数。
In our proposed method, both the objective function and the perfect reconstruction condition are expressed as a quadratic function of the prototype filter coefficient vector. 该方法中，目标函数和完全重构条件均被表示成为原型滤波器系数矢量的二次函数形式。
