Erratum: W. S. Jassim, On the intersection of finitely generated subgroups of free groups (Q1281525)

Let \(F\) be a free group, and let \(H\) and \(K\) be finitely generated subgroups of \(F\). Recently \textit{W. S. Jassim} [ Rev. Mat. Univ. Complut. 9, No. 1, 67--84 (1996; Zbl 0884.20016)] claimed to obtain a certain upper bound for the rank of \(H\cap K\) in terms of the ranks of \(H\) and \(K\). Jassim's proof is not valid, and we will demonstrate this by giving a counterexample to his Lemma 2.4.