# Tenacity and rupture degree of permutation graphs of complete bipartite graphs

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1 Tenacity and rupture degree of permutation graphs of complete bipartite graphs Fengwei Li, Qingfang Ye and Xueliang Li Department of mathematics, Shaoxing University, Shaoxing Zhejiang , P.R. China Center for Combinatorics and LPMC, Nankai University Tianjin , P.R. China Abstract Computer or communication networks are so designed that they don t easily get disrupted under external attack and, moreover, these are easily reconstructed when they do get disrupted. These desirable properties of networks can be measured by various parameters such as connectivity, toughness, tenacity and rupture degree. Among these parameters, tenacity and rupture degree are comparatively better parameters to measure the stability of networks. In this paper, the authors give the exact values for the tenacity and rupture degree of permutation graphs of complete bipartite graphs. Keywords: rupture degree, tenacity, permutation graph. AMS subject classification(2000): 05C40, 05C90, 68R10, 94C15 1. Introduction Throughout this paper, a graph G = (V, E) always means a simple connected graph with vertex set V and edge set E. We use Bondy and Murty supported by the NSFC(No ),PCSIRT and the 973 program. Corresponding author. 1

2 [1] for terminology and notations not defined here. A set of vertices S of G is called a vertex cut set if G S is disconnected. Measures of the vulnerability of graphs are currently of growing interest among graph theorists and network designers. Among vulnerability parameters, much have been done recently on the toughness, binding number of different classes of graphs since these parameters are more sensitive to the structure of the graph than is the connectivity of the graph. In [4], Guichard et al. given the integrity, toughness, and binding number for permutation graphs of complete and complete bipartite graphs. In the following two definitions, m(g S), and ω(g S), respectively, denotes the order of the largest component and number of components in G S. The tenacity of a graph G, T(G), which is defined by Cozzens in [3], is defined as S + m(g S) T(G) = min ω(g S) : S V (G) is a vertex cut set of G}. The rupture degree of a noncomplete graph G, r(g), introduced by Li, Zhang and Li in [6], is defined as r(g) = maxω(g S) S m(g S) : S V (G) is a vertex cut set of G}. In particular, the tenacity and rupture degree of a complete graph K n is defined to be n and 1 n respectively. Clearly, of all the above parameters, tenacity and rupture degree are comparatively appropriate for measuring the vulnerability of networks. Similarly to the relation between the toughness and scattering number, the rupture degree and tenacity also differ in showing the vulnerability of networks. This can be shown as follows. Consider the graphs G 1 and G 2 in Figure 1, It is not difficult to check that T(G 1 ) = T(G 2 ) = 1 2, but r(g 1) = 3 and r(g 2 ) = 4. Clearly r(g 1 ) r(g 2 ). On the other hand, we consider graphs G 3 = K 1 + (K n b 1 E b ) and G 4 = K 2 + (K n b 3 E b+1 ), it is obvious that r(g 3 ) = r(g 4 ), but T(G 3 ) T(G 4 ) unless n = 2b + 1, where b is an integer. Hence rupture degree is a better parameter for distinguishing the vulnerability of these two graphs G 1 and G 2, but the tenacity is a better parameter for distinguishing the vulnerability of these two graphs G 3 and G 4. It is easy to see that the higher the tenacity (the less the rupture degree) of a network the more stable it is considered to be. In [2], the authors introduced permutation graphs and proceeded to characterize those which are planar. 2

3 G 1 G 2 Figure 1. For a graph G with n vertices labelled 1, 2,, n, n 4, and a permutation α S n, the symetric group on the n symbols 1, 2,, n}, the α-permutation graph of G, P α (G) consists of two disjoint copies of G, G x and G y, along with the n edges obtained by join x i in G x with y α(i) in G y, i = 1, 2,, n. It is well known that permutation graphs have high connectivity properties, as is shown in [8] and [9]. As special graphs, some vulnerability parameters of permutation graphs of complete bipartite graphs have been determined in [4]. In [5], we give the following decision problem. Problem 2.1 Not r-rupture Instance: An incomplete connected graph G, and an integer r. Question: Does there exist an X V (G) with ω(g X) 2 such that ω(g X) > X + m(g X) + r? And by this decision problem, we proved that computing the rupture degree of a graph is NP-hard in general and so is the problem of determining the tenacity of a graph [7], so it is an interesting problem to determine these two parameters for some special graphs. In this paper, Formulas for computing the rupture degree and tenacity for permutation graphs of complete bipartite graphs are determined. 2. Tenacity and rupture degree of the permutation Graphs of complete bipartite graphs In this section, we fix our attention on permutation graph of complete bipartite graph, P α (K m,n ). Assume that m n and that M and N are the sets of the partitions of size m and n respectively. Furthermore, assume that the vertices of M are labelled 1, 2,, m and that vertices of N are labelled m + 1, m + 2,, m + n. For the permutation graph P α (K m,n ), let M x and M y denote the partitions of the first copy of K m,n, N x and N y denote the 3

4 partitions of the second copy of K m,n, and let q denote the number of vertices in M x that are joined by permutation edges to vertices in M y. It is well known that the connectivity, toughness, integrity and the binding number of P α (K m,n ) can be expressed in terms of the parameters m, n, and/or q as follows. Theorem 2.1([4]) For α in S m+n, and m n t(p α (K m,n )) = 2m m+n q if q < n2 +m 2 n+3m n+m n+q if q n2 +m 2 n+3m Theorem 2.2([4]) For α in S m+n, and m n 2m + 1 if m = n and q 0, m} I(P α (K m,n )) = 2m + 2 otherwise. Theorem 2.3 ([2]) For α in S m+n, and m n n+q if q < nm q 2m+n 1 b(p α (K m,n )) = 2m+2n 1 nm if m+2n 1 2n+m 1 q < m2 +3mn 2m 4n+2m 2 3m+n 2q if m2 +3mn 2m q. n+m 4n+2m 2 Theorem 2.4([8]) For α in S m+n, and m n, κ(p α (K m,n ) = m + 1. In the following, we determine the rupture degree and tenacity of the permutation graph of complete bipartite graphs in terms of the parameters m, n and/or q. In the proofs of the remaining theorems we will use the following definitions and observations. Let M x be the set of vertices in M x that are joined by permutation edges to vertices in M y and let M y be these vertices in M y. So M y = M x = q. Let M x = M x M x and M y = M y M y and thus M y = M x = m q. Now the vertices in M x are adjacent to vertices in N y by permutation edges, we call this vertex set N y. Similarly define N x to be the set of vertices in N x adjacent to the vertices in M y by permutation edges. Thus N x = N y = m q. Finally let N x = N x N x and N y = N y N y, so N x = N y = n m + q. Note that since 0 q m, some of these sets may be empty. Let K = M x, M x, M y, M y, N x, N x, N y, N y }. The relationship among these sets in K is shown in Figure 2. Remark: It is easy to see that, when m = n = 1, whether q = 1 or q = 0, the two graphs are isomorphic. So under this condition, we assume that q = 0. To prove our main result we first give a lemma. 4

5 M x M y M x M x M y M y N x N x N y N y N x Figure 2. Relationship among the sets in K Lemma 2.1 For α in S m+n and m n, there exists a vertex cut set S of graph P α (K m,n ) with T(P α (K m,n )) = S +m(pα(km,n) S) ω(p α(k m,n) S), such that for all Z in K, if Z S is not empty, then Z S. Proof. By the symmetry of the permutation graph of complete bipartite graph, We do the case when Z = M x. Let S be the minimum vertex cut set of P α (K m,n ) with T(P α (K m,n )) = S +m(p α(k m,n) S ) ω(p α(k m,n) S and let A ) x = S M x and B x = M x A x. We let A y be the neighborhood of A x in M y, and B y be the neighborhood of B x in M y. Suppose that A x and B x are both nonempty, i.e., m 2. We first note that T = M x M y is a vertex cut set of P α (K m,n ). So by the definition of tenacity, we have T(P α (K m,n )) 2m+2. The proof m+n q proceeds in four cases. Case 1. If N x and N y are both contained in S, let T = S A x, then T = S A x, ω(p α (K m,n ) T) ω(p α (K m,n ) S ), m(p α (K m,n ) T) m(p α (K m,n ) S T +m(pα(km,n) T) ) + 1. So, S +m(p α(k m,n) S ), which con-. Thus, tradicts the minimality of S. N y S +m(p α(k m,n) S ) ( A x 1) T +m(pα(km,n) T) = S +m(p α(k m,n) S ) Case 2. If N x is contained in S but N y is not contained in S, then let x i be an element of A x and so y α(i) is in A y. Let T = S x i }, then T = S 1. If y α(i) is not contained in S, then ω(p α (K m,n ) T) = ω(p α (K m,n ) T +m(p α(k m,n) T) T +m(p α(k m,n) T) = S +m(p α(k m,n) S ) S ), m(p α (K m,n ) T) m(p α (K m,n ) S ) + 1. So, S +m(p α(k m,n) S ). Thus,, but T has one less vertex than that of S, a contradiction. If y α(i) is contained in S, then ω(p α (K m,n ) T) = ω(p α (K m,n ) S )+1, m(p α (K m,n ) T) = m(p α (K m,n ) S T +m(pα(km,n) T) ). So, = S 1+m(P α(k m,n) S ) ω(p α(k m,n) S < S +m(p α(k m,n) S ) )+1 ω(p α(k m,n) S, again ) a contradiction. 5

6 Case 3. If N x is not contained in S but N y is contained in S, then let x i be in A x and so y α(i) is in A y. If y α(i) is in S, let T = S x i }, then T = S 1, ω(p α (K m,n ) T) = ω(p α (K m,n ) S ), m(p α (K m,n ) T) = m(p α (K m,n ) S T +m(p ) + 1. So, α(k m,n) T) = S +m(p α(k m,n) S ) ω(p α(k m,n) S, but T ) has one less vertex than that of S, a contradiction. Hence A y S is empty. Now let x i be in B x, then y α(i) is contained in B y. If y α(i) is in S, then let T = S y α(i) } x i }. Thus, T = S, ω(p α (K m,n ) T) ω(p α (K m,n ) S ) + 1, m(p α (K m,n ) T) m(p α (K m,n ) S ) 1. So, T +m(p α(k m,n) T) < S +m(p α(k m,n) S ), a contradiction. So B y S is empty. Thus M y S is empty. Let T = S B x, then T = S + B x, ω(p α (K m,n ) T) ω(p α (K m,n ) S )+ B x, m(p α (K m,n ) T) m(p α (K m,n ) S ) B x. T +m(pα(km,n) T) S +m(p α(k m,n) S ), this contra- So dicts the definition of Tenacity. + B x < S +m(p α(k m,n) S ) Case 4. If N x and N y are not contained in S, then consider M y. If M y is contained in S, let T = S B x, then T = S + B x, ω(p α (K m,n ) T) ω(p α (K m,n ) S ), m(p α (K m,n ) T) m(p α (K m,n ) S ) B x. T +m(pα(km,n) T) So,. Thus T is a vertex cut set with S +m(p α(k m,n) S ) T +m(p α(k m,n) T) = S +m(p α(k m,n) S ), and M x T. If M y is not contained in S, then it is easy to see that all of the vertices in N x S are in the same component since B x is nonempty, and all of the vertices in N y S are in the same component since M y S is nonempty. Thus P α (K m,n ) S has exactly two components, one in each copy of K m,n. If neither x i nor y α(i) is not in S then S is not a cut set. Thus at least one of x i and y α(i) is in S for all i = 1, 2,, n+m. Thus we know that S n+m, ω(p α (K m,n ) S ) = 2. Let C be the component of P α (K m,n ) S containing B x, then T(P α (K m,n )) = S +m(p α(k m,n) S ) S + V (C) S + B x + N x S n+m+2 ω(p α(k m,n) S n+m+2 ) 2 m+n q 2m+2. m+n q On the other hand, by the previous remark we know that T(P α (K m,n )) 2m+2. Hence, in this case T(P m+n q α(k m,n )) = 2m+2. Let T = M m+n q x M y. Then T is a vertex cut set with T(P α (K m,n )) = in T. T +m(pα(km,n) T) and M x is contained From above we know that the lemma is true if Z = M x. The above proof works for the other cases of Z K. The details are omitted. By the above lemma we can obtain the tenacity of the permutation graph of complete bipartite graph. Theorem 2.4 For α in S m+n and m n (1) if 1 = m = n, T(P α (K m,n )) = 3 2. (2) if 1 = m < n, when q = 0, T(P α (K m,n )) = 4 n+1. 6

7 when q = 1, (3) if 2 m n, when q = 0, T(P α (K m,n )) = T(P α (K m,n )) = n+2 n+1 if 2 n 3 4 n if n > 3. 2m+1 m+n if m = n 2m+2 m+n if m < n. when q = m, T(P α (K m,n )) = 2m+2 n n+m+1 n+m if m < m2 +n 2 +m n 3m+n+3 if m m2 +n 2 +m n. 3m+n+3 when 1 q m 1, T(P α (K m,n )) = 2m+2 m+n q if q < m2 +2n+n 2 3m+n+4 n+m+2 n+q if q m2 +2n+n 2 3m+n+4 Proof. By Lemma 2.1 we know that the vertex set satisfying the condition must be the union of the elements of K. It is easy to find 55 vertex cut sets of this type. But most of these sets are trivial, all but 4 of these sets may be discarded as giving too larger values for S +m(pα(km,n) S) ω(p α(k m,n) S) sets are S 1 = M x M y, S 2 = M x M y, S 3 = M x N y, S 4 = M x N y values for S +m(pα(km,n) S) ω(p α(k m,n) S) v 3 = m+n+2 q+n if q m or v 3 = m+n+1 q+n. The remaining, and the given by these sets are v 1 = m+2n+q m q+1, v 2 = 2m+2 if q = m, v 4 = 3m+n q n m+q+1 three cases. Case 1. When m = n = 1, it is easy to see that (a) if q = 0, T(P α (K m,n )) = minv 1,, v 4 } = v 2 = 3 2. (b) if q = 1, T(P α (K m,n )) = minv 1,, v 4 } = v 4 = 3 2. So under this condition T(P α (K m,n )) = 3 2. Case 2. When 1 = m < n, n+m q,. We distinguish Subcase 2.1 if q = 0, then T(P α (K m,n )) = minv 1,, v 4 } = v 2 = 4 n+1. Subcase 2.2 if q = 1, then (a) when 2 n 3, T(P α (K m,n )) = minv 1,, v 4 } = v 4 = n+2. n+1 (b) when n > 3, T(P α (K m,n )) = minv 1,, v 4 } = v 2 = 4. n Case 3. When 2 m n, Subcase 3.1 if q = 0, (a) when m = n, T(P α (K m,n )) = minv 1,, v 4 } = v 2 = m+n+1. m+n. m+n (b) when m < n, T(P α (K m,n )) = minv 1,, v 4 } = v 2 = m+n+2 Subcase 3.2 if 1 q m, for fixed m and n, when q increases, the following 7

8 occur. The value v 1 increases, so the minimum value of v 1 is m+2n+1. v m 4 decreases, so the minimum value of v 4 is 2m+n. When q m increases, The n+1 value v 3 decreases, so the maximum value of v 3 is m+n+2. It is easy to check n+1 that the minimum value of v 1 is larger than the maximum value of v 2, and the minimum value of v 4 is larger than the maximum value of v 3. And it is also easily checked that when q = m, the minimum value of v 4 is larger than the value of v 3 = m+n+1. So S q+n 1 and S 4 should be discarded. Now the value of v 3 decreases as q increases, and the intersection point for v 2 and v 3 occurs where q = m2 +2n+n 2, when v 3m+n+4 3 = m+n+2 and where q = m2 +n 2 +m n, q+n 3m+n+3 when v 3 = m+n+1. Thus the theorem holds. q+n The following theorem gives us the rupture degree of permutation graph of complete bipartite graphs. Note that T = M x M y is a vertex cut set of P α (K m,n ), so by the definition of the rupture degree we have r(p α (K m,n )) ω(p α (K m,n ) T) T m(p α (K m,n ) T) n m q 2. In order to prove this theorem we first introduce a lemma. Lemma 2.2 For α in S m+n and m n, there exists a vertex cut set S of P α (K m,n ) with r(p α (K m,n )) = ω(p α (K m,n ) S) S m(p α (K m,n ) S) such that for all Z in K, if Z S is not empty, then Z S. Proof. By the symmetry of the permutation graph of complete bipartite graph, We do the case when Z = M x. Let S be the minimum vertex cut set of P α (K m,n ) with r(p α (K m,n )) = ω(p α (K m,n ) S ) S m(p α (K m,n ) S ), and let A x = S M x and B x = M x A x. Suppose that A x and B x are both nonempty, i.e., m 2. We distinguish four cases. Case 1. If N x and N y are both contained in S, let T = S A x, then T = S A x, ω(p α (K m,n ) T) ω(p α (K m,n ) S ), m(p α (K m,n ) T) m(p α (K m,n ) S ) + 1. So, ω(p α (K m,n ) T) T m(p α (K m,n ) T) ω(p α (K m,n ) S ) S m(p α (K m,n ) S ) + A x 1 ω(p α (K m,n ) S ) S m(p α (K m,n ) S ), a contradiction to the minimality of S. Case 2. If N x is contained in S but N y is not contained in S, let T = S A x, then T = S A x, ω(p α (K m,n ) T) ω(p α (K m,n ) S ), m(p α (K m,n ) T) m(p α (K m,n ) S ) + A x. So, ω(p α (K m,n ) T) T m(p α (K m,n ) T) ω(p α (K m,n ) S S m(p α (K m,n ) S ), which contradicts the minimality of S. Case 3. If N x is not contained in S but N y is contained in S, then M y S is empty, the proof is similar to that of Case 3 in Lemma 2.1. Let T = S B x. Thus T = S + B x, ω(p α (K m,n ) T) ω(p α (K m,n ) S )+ B x, 8

9 m(p α (K m,n ) T) m(p α (K m,n ) S ) B x. So, ω(p α (K m,n ) T) T m(p α (K m,n ) T) ω(p α (K m,n ) S ) S m(p α (K m,n ) S ) + B x > ω(p α (K m,n ) S ) S m(p α (K m,n ) S ), which contradicts the definition of rupture degree. Case 4. If neither N x nor N y is contained in S, then consider M y. If M y is contained in S, let T = S B x, then T = S + B x, ω(p α (K m,n ) T) ω(p α (K m,n ) S ), m(p α (K m,n ) T) m(p α (K m,n ) S ) B x. So, ω(p α (K m,n ) T) T m(p α (K m,n ) T) ω(p α (K m,n ) S ) S m(p α (K m,n ) S ). Thus T is a vertex cut set with ω(p α (K m,n ) T) T m(p α (K m,n ) T) = ω(p α (K m,n ) S ) S m(p α (K m,n ) S ), and M x T. If M y is not contained in S, then it is easy to see that all of the vertices in N x S are in the same component since B x is nonempty, and all of the vertices in N y S are in the same component since M y S is nonempty. Thus P α (K m,n ) S has exactly two components, one in each copy of K m,n. If neither x i nor y α(i) is not in S, then S is not a cut set. Thus at least one of x i and y α(i) is in S for all i = 1, 2,, n + m. Thus we know that S n+m, ω(p α (K m,n ) S ) = 2. Let C be the component of P α (K m,n ) S containing B x, then r(p α (K m,n )) = ω(p α (K m,n ) S ) S m(p α (K m,n ) S ) ω(p α (K m,n ) S ) S V (C) ω(p α (K m,n ) S ) S B x N x S 2 n m 2 n m q 2. On the other hand, by the previous remark we know that r(p α (K m,n )) n m q 2. Hence, in this case r(p α (K m,n )) = n m q 2. Let T = M x M y. Then T is a cut set with r(p α (K m,n )) = ω(p α (K m,n ) T) T m(p α (K m,n ) T) and M x T. From above we know that the lemma is true if Z = M x. The above proof works for the other cases of Z K. The details are omitted. Theorem 2.5 For α in S m+n and m n (1) if m = n = 1, r(p α (K m,n )) = 1. (2) if 1 = m < n, when q = 0, r(p α (K m,n )) = n 3. when q = 1, 1 if 2 n 3 r(p α (K m,n )) = n 4 if n > 3. (3) if 2 m n, when q = 0, r(p α (K m,n )) = n m 2 if n m 2 1 if n m < 2. 9

10 when q = m, r(p α (K m,n )) = n 2m 2 1 if q n 1 2 if q > n 1. 2 when 1 q m 1, r(p α (K m,n )) = n m q 2 if q n 2 q m 2 if q > n. 2 Proof. By Lemma 2.2 we know that the vertex set satisfying the condition must be the union of the elements of K. It is easy to find 55 vertex cut sets of this type. But most of these sets are trivial, all but 5 of these sets may be discarded as giving too less values for ω(p α (K m,n ) S) S m(p α (K m,n ) S). The remaining sets are S 1 = M x M y, S 2 = M x N y, S 3 = M x M y N x, S 4 = M x N y, S 5 = M x M y N y and the values for ω(p α(k m,n ) S) S m(p α (K m,n ) S) given by these sets are v 1 = n m q 2, v 2 = q m 2 if q m or v 2 = 1 if q = m, v 3 = 2q 1, v 4 = 4m+2q +1, v 5 = 2m+1. So we distinguish three cases. Case 1. When m = n = 1, it is easy to see that r(p α (K m,n )) = maxv 1, v 2,, v 5 } = v 5 = 1. Case 2. When 1 = m < n, Subcase 2.1 If q = 0, it is easy to see that r(p α (K m,n )) = maxv 1, v 2,, v 5 } = v 1 = n 3. Subcase 2.2 If q = 1, it is easy to see that (a) when n = 2, then r(p α (K m,n )) = maxv 1, v 2,, v 5 } = v 5 = 1. (b) when n > 2, then r(p α (K m,n )) = maxv 1, v 2,, v 5 } = v 1 = n 4. Case 3. When 2 m n, Subcase 3.1 If q = 0, it is easy to see that (a) when n m 2, then r(p α (K m,n )) = maxv 1, v 2,, v 5 } = v 1 = n m 2. (b) when n m < 2, then r(p α (K m,n )) = maxv 1, v 2,, v 5 } = v 3 = 1. Subcase 3.2 If q = m, then, when q n 1, maxv 2 1, v 2,, v 5 } = v 1 = n 2m 2; when q n 1, maxv 2 1, v 2,, v 5 } = v 2 = 1. Subcase 3.3 If 1 q m 1, under this condition, for fixed m and n, when q n, then maxv 2 1, v 2,, v 5 } = v 1 = n m q 2; when q n, 2 maxv 1, v 2,, v 5 } = v 2 = q m 2. The proof is thus completed. 10

11 3. Conclusion The rupture degree and tenacity of a graph, to some extent, represents a trade-off between the amount of work done to damage the network and how badly the network is damaged. Hence, the rupture degree and tenacity can be used to measure the vulnerability of networks. So clearly, it is of prime importance to determine this parameter for a graph. In this paper, we have obtained the exact values for the rupture degree and tenacity of permutation graphs of complete bipartite graphs. To make further progress in this direction, one could try to characterize the graphs with given rupture degree or tenacity. Acknowledgement This work was supported by the NSFC(No ),PCSIRT and the 973 program. The authors are thankful to anonymous referees for their useful comments, which led to this improved version. References [1] J. A. Bondy and U. S. R. Murty, Graph Theory with Applications, Macmillan, London and Elsevier, New york, [2] G. Chartrand and F. Harary, Planar permutation graphs, Ann. Inst. H. Poincare Sec. B (N.S.) 3(1967), [3] M. Cozzens, D. Moazzami and S. Stueckle, The tenacity of a graph, Proc. 7th International Conference on the Theory and Applications of Graphs, Wiley, New York, 1995, [4] D. Guichard, B. Piazza and S. Stueckle, On the vulnerability of permutation graphs of complete and complete bipartite graphs, Ars Combinatoria 31(1991), [5] F.W. Li and X.L. Li, Computing the rupture degrees of graphs, Proc. 7th international symposium on parallel architectures, algorithms and networks, IEEE computer society, Los Alamitos, California, 2004, [6] Y. K. Li, S. G. Zhang and X. L. Li, Rupture degree of graphs, Int. J. Comput. Math.,82(7)(2005),

12 [7] D. E. Mann, The tenacity of trees, Ph.D.Thesis, Northeastern University, [8] B. Piazza, Connectivity and hamilitonian properties of permutation graphs, Ph.D.Dissertation, Clemsom University, August [9] B. Piazza and S. Stueckle, On the cut frequency vector of permutation graphs, Congr.Numer. 49(1985), [10] A. Kirlangic, The Rupture Degree and Gear Graphs, Bull. Malays.Math. Sci. Soc. (2) 32(1) (2009),

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