direct graphs的用法和樣例：
例句
- And the coefficients of eigen-polynomial expressions were closely related to the circular sets of a directional graph.
而特徵多項式的係數與有向圖的圓族有著密切的關係。 - In a directed graph, the decision variables of the shortest path computation must be determined by a constraint that is called Kirchoff's constraint.
摘要已知一個有向圖，求解兩點間的最短路徑的決策變數滿足基爾霍夫約束關係。
