Lemma 26.1 Let G =(V.E)be a flow network with source s and sink t,and let f be a flow in G.Let Gf be the residual network of G induced by f,and let f be a flow in Gf.Then the function ft f defined in equation (26.4)is a flow in G with value ff=f+fl. 证明要点： 1,证明函数是一个流：符合流的两个特性 2,证明f↑f1=|f1+If"证明要点： 1，证明函数是一个流：符合流的两个特性 2，证明