directivity function的用法和樣例：
例句
- The equivalence between the optimality function and the first order optimal condition is discussed.
論述了最優性函數與一階最優性條件的等價性。 - The optimization algorithm for the relaxed subproblem is constructed by virtue of the optimality function and the convergence is proved.
利用最優性函數構造了鬆弛子問題的優化演算法，並證明了演算法的收斂性。
