directivity function的用法和样例：
例句
- The equivalence between the optimality function and the first order optimal condition is discussed.
论述了最优性函数与一阶最优性条件的等价性。 - The optimization algorithm for the relaxed subproblem is constructed by virtue of the optimality function and the convergence is proved.
利用最优性函数构造了松弛子问题的优化算法，并证明了算法的收敛性。
