Text book name: Introduction to algorithms, data structures and formal languages. (Mark C Wilson, Michael J. Dinneen, Georgy Gimel'farb)

SOLUTION TO ExERCISE 5.6.2 ON PAGE 109: By Theorem 5.11, every DAG has a topological ordering (vi, v2,..Vn) such that there are no arcs (vi,y) E(G) such that c j. This means that there are no arcs going from right to left in the topological ordering. This means that node vi has no arcs going into it and node vn has no nodes going away from it, they are respectively, a source and sink node. Therefore for every DAG there is at least one source and sink node. SOLUTION TO ExERCISE 5.6.2 ON PAGE 109: By Theorem 5.11, every DAG has a topological ordering (vi, v2,..Vn) such that there are no arcs (vi,y) E(G) such that c j. This means that there are no arcs going from right to left in the topological ordering. This means that node vi has no arcs going into it and node vn has no nodes going away from it, they are respectively, a source and sink node. Therefore for every DAG there is at least one source and sink node