# 10. Graph Matrices Incidence Matrix

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1 10 Graph Matrices Since a graph is completely determined by specifying either its adjacency structure or its incidence structure, these specifications provide far more efficient ways of representing a large or complicated graph than a pictorial representation As computers are more adept at manipulating numbers than at recognising pictures, it is standard practice to communicate the specification of a graph to a computer in matrix form In this chapter, we study various types of matrices associated with a graph, and our study is based on Narsing Deo [63], Foulds [82], Harary [104] and Parthasarathy [180] 101 Incidence Matrix Let G be a graph with n vertices, m edges and without self-loops The incidence matrix A of G is an n m matrix A = [a i j ] whose n rows correspond to the n vertices and the m columns correspond to m edges such that { 1, i f jth edge m a i j = j is incident on the ith vertex 0, otherwise It is also called vertex-edge incidence matrix and is denoted by A(G) Example Consider the graphs given in Figure 101 The incidence matrix of G 1 is A(G 1 ) = v 1 v 2 v 3 v 4 v 5 v 6 e 1 e 2 e 3 e 4 e 5 e 6 e 7 e The incidence matrix of G 2 is

2 264 Graph Matrices A(G 2 ) = v 1 v 2 v 3 v 4 e 1 e 2 e 3 e 4 e The incidence matrix of G 3 is A(G 3 ) = v 1 v 2 v 3 v 4 e 1 e 2 e 3 e 4 e Fig 101 The incidence matrix contains only two types of elements, 0 and 1 This clearly is a binary matrix or a (0, 1)-matrix We have the following observations about the incidence matrix A 1 Since every edge is incident on exactly two vertices, each column of A has exactly two one s 2 The number of one s in each row equals the degree of the corresponding vertex

3 Graph Theory A row with all zeros represents an isolated vertex 4 Parallel edges in a graph produce identical columns in its incidence matrix 5 If a graph is disconnected and consists of two components G 1 and G 2, the incidence matrix A(G) of graph G can be written in a block diagonal form as [ A(G1 ) 0 A(G) = 0 A(G 2 ) ], where A(G 1 ) and A(G 2 ) are the incidence matrices of components G 1 and G 2 This observation results from the fact that no edge in G 1 is incident on vertices of G 2 and vice versa Obviously, this is also true for a disconnected graph with any number of components 6 Permutation of any two rows or columns in an incidence matrix simply corresponds to relabeling the vertices and edges of the same graph Note The matrix A has been defined over a field, Galois field modulo 2 or GF(2), that is, the set {0, 1} with operation addition modulo 2 written as + such that 0+0 = 0, 1+0 = 1, 1+1= 0 and multiplication modulo 2 written as such that 00 = 0, 10= 0 = 01, 11=1 The following result is an immediate consequence of the above observations Theorem 101 Two graphs G 1 and G 2 are isomorphic if and only if their incidence matrices A(G 1 ) and A(G 2 ) differ only by permutation of rows and columns Proof Let the graphs G 1 and G 2 be isomorphic Then there is a one-one correspondence between the vertices and edges in G 1 and G 2 such that the incidence relation is preserved Thus A(G 1 ) and A(G 2 ) are either same or differ only by permutation of rows and columns The converse follows, since permutation of any two rows or columns in an incidence matrix simply corresponds to relabeling the vertices and edges of the same graph Rank of the incidence matrix Let G be a graph and let A(G) be its incidence matrix Now each row in A(G) is a vector over GF(2) in the vector space of graph G Let the row vectors be denoted by A 1, A 2,, A n Then, A(G) = A 1 A 2 A n

4 266 Graph Matrices Since there are exactly two ones in every column of A, the sum of all these vectors is 0 (this being a modulo 2 sum of the corresponding entries) Thus vectors A 1, A 2,, A n are linearly dependent Therefore, rank A < n Hence, rank A n 1 From the above observations, we have the following result Theorem 102 If A(G) is an incidence matrix of a connected graph G with n vertices, then rank of A(G) is n 1 Proof Let G be a connected graph with n vertices and let the number of edges in G be m Let A(G) be the incidence matrix and let A 1, A 2,, A n be the row vector of A(G) Then, A(G) = A 1 A 2 A n (1021) Clearly, rank A(G) n 1 (1022) Consider the sum of any m of these row vectors, m n 1 Since G is connected, A(G) cannot be partitioned in the form [ A(G1 ) 0 A(G) = 0 A(G 2 ) ] such that A(G 1 ) has m rows and A(G 2 ) has n m rows Thus there exists no m m submatrix of A(G) for m n 1, such that the modulo 2 sum of these m rows is equal to zero As there are only two elements 0 and 1 in this field, the additions of all vectors taken m at a time for m = 1, 2,, n 1 gives all possible linear combinations of n 1 row vectors Thus no linear combinations of m row vectors of A, for m n 1, is zero Therefore, rank A(G) n 1 (1023) Combining (1022) and (1023), it follows that rank A(G) = n 1 Remark If G is a disconnected graph with k components, then it follows from the above theorem that rank of A(G) is n k Let G be a connected graph with n vertices and m edges Then the order of the incidence matrix A(G) is n m Now, if we remove any one row from A(G), the remaining (n 1) by m submatrix is of rank (n 1) Thus the remaining (n 1) row vectors are linearly independent This shows that only (n 1) rows of an incidence matrix are required to specify the

5 Graph Theory 267 corresponding graph completely, because (n 1) rows contain the same information as the entire matrix This follows from the fact that given (n 1) rows, we can construct the nth row, as each column in the matrix has exactly two ones Such an (n 1) m matrix of A is called a reduced incidence matrix and is denoted by A f The vertex corresponding to the deleted row in A f is called the reference vertex Obviously, any vertex of a connected graph can be treated as the reference vertex The following result gives the nature of the incidence matrix of a tree Theorem 103 The reduced incidence matrix of a tree is non-singular Proof A tree with n vertices has n 1 edges and also a tree is connected Therefore, the reduced incidence matrix is a square matrix of order n 1, with rank n 1 Thus the result follows Now a graph G with n vertices and n 1 edges which is not a tree is obviously disconnected Therefore the rank of the incidence matrix of G is less than (n 1) Hence the (n 1) (n 1) reduced incidence matrix of a graph is non-singular if and only if the graph is a tree 102 Submatrices of A(G) Let H be a subgraph of a graph G, and let A(H) and A(G) be the incidence matrices of H and G respectively Clearly, A(H) is a submatrix of A(G), possibly with rows or columns permuted We observe that there is a one-one correspondence between each n k submatrix of A(G) and a subgraph of G with k edges, k being a positive integer, k < m and n being the number of vertices in G The following is a property of the submatrices of A(G) Theorem 104 Let A(G) be the incidence matrix of a connected graph G with n vertices An (n 1) (n 1) submatrix of A(G) is non-singular if and only if the n 1 edges corresponding to the n 1 columns of this matrix constitutes a spanning tree in G Proof Let G be a connected graph with n vertices and m edges So, m n 1 Let A(G) be the incidence matrix of G, so that A(G) has n rows and m columns (m n 1) We know every square submatrix of order (n 1) (n 1) in A(G) is the reduced incidence matrix of some subgraph H in G with n 1 edges, and vice versa We also know that a square submatrix of A(G) is non-singular if and only if the corresponding subgraph is a tree Obviously, the tree is a spanning tree because it contains n 1 edges of the n-vertex graph Hence (n 1) (n 1) submatrix of A(G) is non-singular if and only if n 1 edges corresponding to n 1 columns of this matrix forms a spanning tree

6 268 Graph Matrices The following is another form of incidence matrix Definition: The matrix F = [ f i j ] of the graph G = (V, E) with V = {v 1, v 2,, v n } and E = {e 1, e 2,, e m }, is the n m matrix associated with a chosen orientation of the edges of G in which for each e = (v i, v j ), one of v i or v j is taken as positive end and the other as negative end, and is defined by 1, i f v i is the positive end o f e j, f i j = 1, i f v i is the negative end o f e j, 0, i f v i is not incident with e j This matrix F can also be obtained from the incidence matrix A by changing either of the two 1s to 1 in each column The above arguments amount to arbitrarily orienting the edges of G, and F is then the incidence matrix of the oriented graph The matrix F is then the modified definition of the incidence matrix A Example Consider the graph G shown in Figure 102, with V = {v 1, v 2, v 3, v 4 } and E = {e 1, e 2, e 3, e 4, e 5 } The incidence matrix is given by A = v 1 v 2 v 3 v 4 e 1 e 2 e 3 e 4 e Therefore, F = v 1 v 2 v 3 v 4 e 1 e 2 e 3 e 4 e Fig 102

7 Graph Theory 269 Theorem 105 If G is a connected graph with n vertices, then rank F = n 1 Proof Let G be a connected graph with V = {v 1, v 2,, v n } and E = {e 1, e 2,, e m } Then the matrix F = [ f i j ] n m is given by f i j = 1, i f v i is the positive end o f e j, 1, i f v i is the negative end o f e j, 0, i f v i is not incident with e j Let R j be the jth row of F Since each column of F has only one +1 and one 1, as non-zero entries, rank F < n Thus, rank F n 1 Now, let n c j R j = 0 be any other linear dependence relation of R 1, R 2,, R n with at least 1 one c j non-zero If c r 0, then the row R r has non-zero entries in those columns which correspond to edges incident with v r For each such column there is just one row, say R s, at which there is a non-zero entry (with opposite sign to the non-zero entry in R r ) The dependence relation thus requires c s = c r, for all s corresponding to vertices adjacent to v r Since G is( connected, ) n we have c j = c, for all j = 1, 2,, n Therefore the dependence relation is c R j = 0, which is same as the first one Hence, rank F = n 1 Alternative Proof Then F = R 1 R 2 M R n (1051) n Since each column of F has only one +1 and one 1 as non-zero entries, R j = 0 j=1 Thus, rank F n 1 (1052) Consider the sum of any m of[ these row] vectors, m n 1 As G is connected, F cannot F1 0 be partitioned in the form F =, such that F 0 F 1 has m rows and F 2 as n m rows 2 Therefore there exists no m m submatrix of F for m n 1, such that the sum of these m rows is equal to zero Therefore, m j=1 R j 0 (1053) 1

8 270 Graph Matrices Also, there is no linear combination of m(m n 1) vectors of F, which is zero For, if m c j R j = 0 is a linear combination, with at least one c j non-zero, say c r 0, then the row j=1 R r has non-zero entries in those columns which correspond to edges incident with v r So for each such column there is just one row, say R s at which there is a non-zero entry with opposite sign to the non-zero entry in R r The linear combination thus requires c s = c r for all s corresponding to vertices adjacent to v r As G is connected, we have c j = c, for all j = 1,, m Therefore the linear combination becomes c( m R j ) = 0, or m R j = 0, which j=1 j=1 contradicts (1053) Hence, rank F = n 1 Theorem 106 If G is a disconnected graph with k components, then rank F = n k Proof F = Since G has k components, F can be partitioned as F F F F k, where F i is the matrix of the ith component G i of G We have proved that rank F i = n i 1, where n i is the number of vertices in G i Thus, rank F = n 1 1+n n k 1 = n 1 + n 2 ++ n k k = n k, as the number of vertices in G is n 1 + n 2 ++ n k = n Corollary 101 A basis for the row space of F is obtained by taking for each i, 1 i k, any n i 1 rows of F i Theorem 107 The determinant of any square submatrix of the matrix F of a graph G has value 1, 1, or zero Proof Let N be the square submatrix of F such that N has both non-zero entries +1 and 1 in each column Then row sum of N is zero and hence N = 0 Clearly if N has no non-zero entries, then N = 0 Now let some column of N have only one non-zero entry Then expanding N with the help of this column, we get N = ± N, where N is a matrix obtained by omitting a row and column of N Continuing in this way, we either get a matrix whose determinant is zero, or end up with a single non-zero entry of N, in which case N = ±1

9 Graph Theory 271 Theorem 108 Let X be any set of n 1 edges of the connected graph G = (V, E) and F x the (n 1) (n 1) submatrix of the matrix F of G, determined by any n 1 rows and those columns which correspond to the edges of X Then F x is non-singular if and only if the edge induced subgraph < X > of G is a spanning tree of G Proof Let F be the matrix corresponding to < X > If < X > is a spanning tree of G, then F x consists of n 1 rows of F Since < X > is connected, therefore rank F x = n 1 Hence F x is non-singular Conversely, let F x be non-singular Then F contains an (n 1) (n 1) non singular submatrix Therefore, rank F = n 1 Since rank + nullity = m, for any graph G, and m(< X >) = n 1 and rank (< X >) = n 1, therefore, nullity (< X >) = 0 Thus < X > is acyclic and connected and so is a spanning tree of G 103 Cycle Matrix Let the graph G have m edges and let q be the number of different cycles in G The cycle matrix B = [b i j ] q m of G is a (0, 1) matrix of order q m, with b i j = 1, if the ith cycle includes jth edge and b i j = 0, otherwise The cycle matrix B of a graph G is denoted by B(G) Example Consider the graph G 1 given in Figure 103 Fig 103 The graph G 1 has four different cycles Z 1 = {e 1, e 2 }, Z 2 = {e 3, e 5, e 7 }, Z 3 = {e 4, e 6, e 7 } and Z 4 = {e 3, e 4, e 6, e 5 } The cycle matrix is

10 272 Graph Matrices B(G 1 ) = z 1 z 2 z 3 z 4 e 1 e 2 e 3 e 4 e 5 e 6 e 7 e The graph G 2 of Figure 103 has seven different cycles, namely, Z 1 = {e 1, e 2 }, Z 2 = {e 2, e 7, e 8 }, Z 3 = {e 1, e 7, e 8 }, Z 4 = {e 4, e 5, e 6, e 7 }, Z 5 = {e 2, e 4, e 5, e 6, e 8 }, Z 6 = {e 1, e 4, e 5, e 6, e 8 } and Z 7 = {e 9 } The cycle matrix is given by B(G 2 ) = z 1 z 2 z 3 z 4 z 5 z 6 z 7 e 1 e 2 e 3 e 4 e 5 e 6 e 7 e 8 e We have the following observations regarding the cycle matrix B(G) of a graph G 1 A column of all zeros corresponds to a non cycle edge, that is, an edge which does not belong to any cycle 2 Each row of B(G) is a cycle vector 3 A cycle matrix has the property of representing a self-loop and the corresponding row has a single one 4 The number of ones in a row is equal to the number of edges in the corresponding cycle 5 If the graph G is separable (or disconnected) and consists of two blocks (or components) H 1 and H 2, then the cycle matrix B(G) can be written in a block-diagonal form as [ B(H1 ) 0 B(G) = 0 B(H 2 ) ], where B(H 1 ) and B(H 2 ) are the cycle matrices of H 1 and H 2 This follows from the fact that cycles in H 1 have no edges belonging to H 2 and vice versa 6 Permutation of any two rows or columns in a cycle matrix corresponds to relabeling the cycles and the edges

11 Graph Theory We know two graphs G 1 and G 2 are 2-isomorphic if and only if they have cycle correspondence Thus two graphs G 1 and G 2 have the same cycle matrix if and only if G 1 and G 2 are 2-isomorphic This implies that the cycle matrix does not specify a graph completely, but only specifies the graph within 2-isomorphism For example, the two graphs given in Figure 104 have the same cycle matrix They are 2-isomorphic, but are not isomorphic Fig 104 The following result relates the incidence and cycle matrix of a graph without self-loops Theorem 109 If G is a graph without self-loops, with incidence matrix A and cycle matrix B whose columns are arranged using the same order of edges, then every row of B is orthogonal to every row of A, that is AB T = BA T 0 (mod2), where A T and B T are the transposes of A and B respectively Proof Let G be a graph without self-loops, and let A and B, respectively, be the incidence and cycle matrix of G We know that in G for any vertex v i and for any cycle Z j, either v i Z j or v i / Z j In case v i / Z j, then there is no edge of Z j which is incident on v i and if v i Z j, then there are exactly two edges of Z j which are incident on v i Now, consider the ith row of A and the jth row of B (which is the jth column of B T ) Since the edges are arranged in the same order, the rth entries in these two rows are both non-zero if and only if the edge e r is incident on the ith vertex v i and is also in the jth cycle Z j We have [AB T ] i j = [A] ir [B T ] r j = [A] ir [B] jr = a ir b jr For each e r of G, we have one of the following cases i e r is incident on v i and e r / Z j Here a ir = 1, b jr = 0 ii e r is not incident on v i and e r Z j In this case, a ir = 0, b jr = 1 iii e r is not incident on v i and e r / Z j, so that a ir = 0, b jr = 0 All these cases imply that the ith vertex v i is not in the jth cycle Z j and we have [AB T ] i j = 0 0 (mod2) iv e r is incident on v i and e r Z j

12 274 Graph Matrices Here we have exactly two edges, say e r and e t incident on v i so that a ir = 1, a it = 1, b jr = 1, b jt = 1 Therefore, [AB T ] i j = a ir b jr = (mod2) We illustrate the above theorem with the following example (Fig 105) Clearly, AB T = = (mod2) Fig We know that a set of fundamental cycles (or basic cycles) with respect to any spanning tree in a connected graph are the only independent cycles in a graph The remaining cycles can be obtained as ring sums (ie, linear combinations) of these cycles Thus, in a cycle matrix, if we take only those rows that correspond to a set of fundamental cycles and remove all other rows, we do not lose any information The removed rows can be formed from the rows corresponding to the set of fundamental cycles For example, in the cycle matrix of the graph given in Figure 106, the fourth row is simply the mod 2 sum of the second and the third rows Fundamental cycles are

13 Graph Theory 275 Z 1 = {e 1, e 2, e 4, e 7 } Z 2 = {e 3, e 4, e 7 } Z 3 = {e 5, e 6, e 7 } Fig 106 Z 1 Z 2 Z 3 e 2 e 3 e 6 e 1 e 4 e 5 e A submatrix of a cycle matrix in which all rows correspond to a set of fundamental cycles is called a fundamental cycle matrix B f The permutation of rows and/or columns do not affect B f If n is the number of vertices, m the number of edges in a connected graph G, then B f is an (m n+1) m matrix because the number of fundamental cycles is m n + 1, each fundamental cycle being produced by one chord Now, arranging the columns in B f such that all the m n + 1 chords correspond to the first m n+1 columns and rearranging the rows such that the first row corresponds to the fundamental cycle made by the chord in the first column, the second row to the fundamental cycle made by the second, and so on This arrangement is done for the above fundamental cycle matrix A matrix B f thus arranged has the form B f = [I µ : B t ], where I µ is an identity matrix of order µ = m n + 1 and B t is the remaining µ (n 1) submatrix, corresponding to the branches of the spanning tree From equation B f = [I µ : B t ], we have rank B f = µ = m n+1 Since B f is a submatrix of the cycle matrix B, therefore, rank B rank B f and thus, rank B m n+1 The following result gives the rank of the cycle matrix

14 276 Graph Matrices Theorem 1010 If B is a cycle matrix of a connected graph G with n vertices and m edges, then rank B = m n+1 Proof Let A be the incidence matrix of the connected graph G Then AB T 0(mod2) Using Sylvester s theorem (Theorem 1013), we have rank A+ rank B T m so that rank A+ rank B m Therefore, rank B m rank A As rank A = n 1, we get rank B m (n 1) = m n+1 But, rank B m n+1 Combining, we get rank B = m n+1 Theorem 1010 can be generalised in the following form Theorem 1011 If B is a cycle matrix of a disconnected graph G with n vertices, m edges and k components, then rank B = m n+k Proof Let B be the cycle matrix of the disconnected graph G with n vertices, m edges and k components Let the k components be G 1, G 2,, G k with n 1, n 2,, n k vertices and m 1, m 2,, m k edges respectively Then n 1 + n 2 ++ n k = n and m 1 + m 2 ++ m k = m Let B 1, B 2,, B k be the cycle matrices of G 1, G 2,, G k Then B(G) = B 1 (G 1 ) B 2 (G 2 ) B 3 (G 3 ) B k (G k ) We know rank B i = m i n i + 1, for 1 i k Therefore, rank B = rank B 1 ++ rank B k = (m 1 n 1 + 1)++(m k n k + 1) = (m 1 ++ m k ) (n 1 ++ n k )+k = m n+k

15 Graph Theory 277 Definition: Let A be a matrix of order k m, with k < m The major determinant of A is the determinant of the largest square submatrix of A, formed by taking any k columns of A That is, the determinant of any k k square submatrix is called the major determinant of A Let A and B be matrices of orders k m and m k respectively (k < m) If columns i 1, i 2,, i k of B are chosen for a particular major of B, then the corresponding major in A consists of the rows i 1, i 2,, i k in A If A is a square matrix of order n, then AX = 0 has a non trivial solution X 0 if and only if A is singular, that is A = 0 The set of all vectors X that satisfy AX = 0 forms a vector space called the null space of matrix A The rank of the null space is called the nullity of A Further more, rank A+nullityA = n These definitions and the above equation also hold when A is a matrix of order k n, k < n We now give Binet-Cauchy and Sylvester theorems which will be used in the further discussions Theorem 1012 (Binet Cauchy) If A and B are two matrices of the order k m and m k respectively (k < m), then AB = sum of the products of corresponding major determinants of A and B Proof We multiply two (m + k) (m + k) partitioned matrices to get [ ][ ] [ ] Ik A A O O AB =, O I m I m B I m B where I m and I k are identity matrices of order m and k respectively [ ] [ ] A O O AB Therefore, det = det I m B I m B [ ] A O Thus, det(ab) =det (10121) I m B Now apply Cauchy s expansion method to the right side of (10121) and observe that the only non-zero minors of any order in I m are its principal minors of that order Therefore, we see that the Cauchy expansion consists of these minors of order m k multiplied by their cofactors of order k in A and B together Theorem 1013 (Sylvester) If A and B are matrices of order k m and n p respectively, then nullity AB nullity A+ nullity B Proof Since every vector X satisfying BX = 0 also satisfies ABX = 0, therefore we have nullity AB nullity B 0 (10131)

16 278 Graph Matrices Let nullity B = s So there exists a set of s linearly independent vectors {x 1, x 2,, x s } forming a basis of the null space of B Therefore, BX i = 0, for i = 1, 2,, s (10132) Now let nullity AB = s + t Thus there exists a set of t linearly independent vectors [X s+1, X s+2,, X s+t ] such that the set {X 1, X 2,, X s, X s+1,, X s+t } forms a basis for the null space of AB Therefore, ABX i = 0, for i = 1, 2,, s, s+1,, s+t (10133) This implies that out of the s+t vectors X i forming a basis of the null space of AB, the first s vectors are made zero by B and the remaining non-zero BX i s, i = s+1,, s+ t are made zero by A Clearly, the vectors BX s+1,, BX s+t are linearly independent For if b 1 BX s+1 + b 2 BX s+2 ++ b t BX s+t = 0, ie, if B(b 1 X s+1 + b 2 X s+2 ++ b t X s+t ) = 0, then the vector b 1 X s+1 +b 2 X s+2 ++b t X s+t is the null space of B, which is possible only if b 1 = b 2 = = b t = 0 Thus we have seen that there are at least t linearly independent vectors which are made zero by A So, nullity A t Since t = (s+t) s, therefore t = nullity AB nullity B Therefore, nullity AB nullity B nullity A, and so nullity AB nullity A + nullity B (10134) Corollary 102 We know, rank A + nullity A = n, and using this in (10134), we get n rank AB n rank A + n rank B Therefore, rank AB rank A+ rank B n If in above, AB = 0, then rank A + rank B n 104 Cut-Set Matrix Let G be a graph with m edges and q cutsets The cut-set matrix C = [c i j ] q m of G is a (0, 1)-matrix with 1, i f ith cutset contains jth edge, c i j = 0, otherwise

17 Graph Theory 279 Example Consider the graphs shown in Figure 10 7 Fig 107(a) Fig 107(b) In the graph G 1, E = {e 1, e 2, e 3, e 4, e 5, e 6, e 7, e 8 } The cut-sets are c 1 = {e 8 }, c 2 = {e 1, e 2 }, c 3 = {e 3, e 5 }, c 4 = {e 5, e 6, e 7 }, c 5 = {e 3, e 6, e 7 }, c 6 = {e 4, e 6 }, c 7 = {e 3, e 4, e 7 } and c 8 = {e 4, e 5, e 7 } The cut-sets for the graph G 2 are c 1 = {e 1, e 2 }, c 2 = {e 3, e 4 }, c 3 = {e 4, e 5 }, c 4 = {e 1, e 6 }, c 5 = {e 2, e 6 }, c 6 = {e 3, e 5 }, c 7 = {e 1, e 4, c 7 }, c 8 = {e 2, e 3, e 7 } and c 9 = {e 5, e 6, e 7 } Thus the cut-set matrices are given by C(G 1 ) = c 1 c 2 c 3 c 4 c 5 c 6 c 7 c 8 e 1 e 2 e 3 e 4 e 5 e 6 e 7 e , and

18 280 Graph Matrices C(G 2 ) = c 1 c 2 c 3 c 4 c 5 c 6 c 7 c 8 c 9 e 1 e 2 e 3 e 4 e 5 e 6 e We have the following observations about the cut-set matrix C(G) of a graph G 1 The permutation of rows or columns in a cut-set matrix corresponds simply to renaming of the cut-sets and edges respectively 2 Each row in C(G) is a cut-set vector 3 A column with all zeros corresponds to an edge forming a self-loop 4 Parallel edges form identical columns in the cut-set matrix 5 In a non-separable graph, since every set of edges incident on a vertex is a cut-set, therefore every row of incidence matrix A(G) is included as a row in the cut-set matrix C(G) That is, for a non-separable graph G, C(G) contains A(G) For a separable graph, the incidence matrix of each block is contained in the cut-set matrix For example, in the graph G 1 of Figure 107, the incidence matrix of the block {e 3, e 4, e 5, e 6, e 7 } is the 4 5 submatrix of C, left after deleting rows c 1, c 2, c 5, c 8 and columns e 1, e 2, e 8 6 It follows from observation 5, that rank C(G) rank A(G) Therefore, for a connected graph with n vertices, rank C(G) n 1 The following result for connected graphs shows that cutset matrix, incidence matrix and the corresponding graph matrix have the same rank Theorem 1014 If G is a connected graph, then the rank of a cut-set matrix C(G) is equal to the rank of incidence matrix A(G), which equals the rank of graph G Proof Let A(G), B(G) and C(G) be the incidence, cycle and cut-set matrix of the connected graph G Then we have rank C(G) n 1 (10141) Since the number of edges common to a cut-set and a cycle is always even, every row in C is orthogonal to every row in B, provided the edges in both B and C are arranged in the same order

19 Graph Theory 281 Thus, BC T = CB T 0 (mod 2) (10142) Now, applying Sylvester s theorem to equation (10142), we have rank B+ rank C m For a connected graph, we have rank B = m n+1 Therefore, rank C m rank B = m (m n+1) = n 1 So, rank C n 1 (10143) It follows from (10141) and (10143) that rank C = n Fundamental Cut-Set Matrix Let G be a connected graph with n vertices and m edges The fundamental cut-set matrix C f of G is an (n 1) m submatrix of C such that the rows correspond to the set of fundamental cut-sets with respect to some spanning tree Clearly, a fundamental cut-set matrix C f can be partitioned into two submatrices, one of which is an identity matrix I n 1 of order n 1 We have C f = [C c : I n 1 ], where the last n 1 columns forming the identity matrix correspond to the n 1 branches of the spanning tree and the first m n+1 columns forming C c correspond to the chords Example Consider the connected graphs G 1 and G 2 given in Figure 108 The spanning tree is shown with bold lines The fundamental cut-sets of G 1 are c 1, c 2, c 3, c 6 and c 7 while the fundamental cut-sets of G 2 are c 1, c 2, c 3, c 4 and c 7 Fig 108 The fundamental cut-set matrix of G 1 and G 2, respectively are given by

20 282 Graph Matrices C f = e 2 e 3 e 4 e 1 e 5 e 6 e 7 e and C f = c 1 c 2 c 3 c 4 c 7 e 1 e 4 e 2 e 3 e 5 e 6 e Relations between A f, B f and C f Let G be a connected graph and A f, B f and C f be respectively the reduced incidence matrix, the fundamental cycle matrix, and the fundamental cut-set matrix of G We have shown that ] B f = [I µ B t (106i) ] and C f = [C c I n 1, (106ii) where B t denotes the submatrix corresponding to the branches of a spanning tree and C c denotes the submatrix corresponding to the chords Let the spanning tree T in Equations (106i) and (106ii) be the same and let the order of the edges in both equations be same Also, in the reduced incidence matrix A f of size (n 1) m, let the edges (ie, the columns) be arranged in the same order as in B f and C f Partition A f into two submatrices given by ] A f = [A c A t, (106iii) where A t consists of n 1 columns corresponding to the branches of the spanning tree T and A c is the spanning submatrix corresponding to the m n+1 chords

21 Graph Theory 283 Since the columns in A f and B f are arranged in the same order, the equation AB T = BA T = 0(mod 2) gives or A f B T f 0(mod 2), ] [A c A t I µ B T t 0(mod 2), or A c + A t B T f 0(mod 2) (106iv) Since A t is non singular, At 1 by At 1, we have or At 1 A c + At 1 A t Bt T 0(mod 2), A 1 t A c + B T t 0(mod 2) Therefore, A 1 t A c = B T t Since in mod 2 arithmetic 1 = 1, B T t exists Now, premultiplying both sides of equation (106iv) = A 1 t A c (106v) Now as the columns in B f and C f are arranged in the same order, therefore (in mod 2 arithmetic) C f B T f 0(mod 2) in mod 2 arithmetic gives C f B T f = 0 Therefore, ] [C c I n 1 I µ B T t = 0, so that C c + B T t = 0, that is, C c = B T t Thus, C c = B T t (as 1 = 1 in mod 2 arithmetic) Hence, C c = A 1 t A c from (106v) Remarks We make the following observations from the above relations 1 If A or A f is given, we can construct B f and C f starting from an arbitrary spanning tree and its submatrix A t in A f 2 If either B f or C f is given, we can construct the other Therefore, since B f determines a graph within 2-isomorphism, so does C f 3 If either B f and C f is given, then A f in general cannot be determined completely

22 284 Graph Matrices Example Consider the graph G of Figure 109 Fig 109 Let {e 1, e 5, e 6, e 7, e 8 } be the spanning tree e 1 e 2 e 3 e 4 e 5 e 6 e 7 e We have, A = Dropping the sixth row in A, we get e 2 e 3 e 4 e 1 e 5 e 6 e 7 e : : A f = : : = [A c : A t ] : e 2 e 3 e 4 e 1 e 5 e 6 e 7 e : B f = : = [I 3 : B t ] and :

23 Graph Theory 285 e 2 e 3 e 4 e 1 e 5 e 6 e 7 e C f = = [C c : I 5 ] Clearly, B T t = C c We verify A 1 t A c = B T t Now, A t = Therefore, At 1 Ac =, B t = Hence, A 1 t A c = Bt T 107 Path Matrix Let G be a graph with m edges, and u and v be any two vertices in G The path matrix for vertices u and v denoted by P(u, v) = [p i j ] q m, where q is the number of different paths between u and v, is defined as 1, i f jth edge lies in the ith path, p i j = 0, otherwise Clearly, a path matrix is defined for a particular pair of vertices, the rows in P(u, v) correspond to different paths between u and v, and the columns correspond to different edges in G For example, consider the graph in Figure 1010

24 286 Graph Matrices Fig 1010 The different paths between the vertices v 3 and v 4 are p 1 = {e 8, e 5 }, p 2 = {e 8, e 7, e 3 } and p 3 = {e 8, e 6, e 4, e 3 } The path matrix for v 3, v 4 is given by e 1 e 2 e 3 e 4 e 5 e 6 e 7 e 8 P(v 3, v 4 ) = We have the following observations about the path matrix 1 A column of all zeros corresponds to an edge that does not lie in any path between u and v 2 A column of all ones corresponds to an edge that lies in every path between u and v 3 There is no row with all zeros 4 The ring sum of any two rows in P(u, v) corresponds to a cycle or an edge-disjoint union of cycles The next result gives a relation between incidence and path matrix of a graph Theorem 1015 If the columns of the incidence matrix A and the path matrix P(u, v) of a connected graph are arranged in the same order, then under the product (mod 2) AP T (u, v) = M, where M is a matrix having ones in two rows u and v, and the zeros in the remaining n 2 rows Proof Let G be a connected graph and let v k = u and v t = v be any two vertices of G Let A be the incidence matrix and P(u, v) be the path matrix of (u, v) in G

25 Graph Theory 287 Now for any vertex v i in G and for any u v path p j in G, either v i p j or v i / p j If v i / p j, then there is no edge of p j which is incident on v i If v i p j, then either v i is an intermediate vertex of p j, or v i = v k or v t In case v i is an intermediate vertex of p j, then there are exactly two edges of p j which are incident on v i and in case v i = v k or v t, there is exactly one edge of p j which is incident on v i Now consider the ith row of A and the jth row of P (which is the jth column of P T (u, v)) As the edges are arranged in the same order, the rth entries in these two rows are both non zero if and only if the edge e r is incident on the ith vertex v i and is also on the jth path p j Let AP T (u, v) = M = [m i j ] We have, [ AP T ] i j = m [A] ir [P T ] r j r=1 Therefore, m i j = m a ir p jr r=1 For each edge e r of G, we have one of the following cases i e r is incident on v i and e r / p j Here a ir = 1, b jr = 0 ii e r is not incident on v i and e r p j Here a ir = 0, b jr = 1 iii e r is not incident on v i and e r / p j Here a ir = 0, b jr = 0 All these cases imply that the ith vertex v i is not in jth path p j and we have M i j = 0 0(mod 2) (Fig 1011(a)) iv e r is incident on v i and e r p j (Fig 1011(b)) If v i is an intermediate vertex of p j, then there are exactly two edges say e r and e t incident on v i so that a ir = 1, a it = 1, p jr = 1, p jt = 1 Therefore, m i j = 1+1 = 0(mod 2) If v i = v k or v t then the edge e r is incident on either v k or v t So, a kr = 1, p jr = 1, or a tr = 1, p jr = 1 Thus, m k j = Σa ir p jr = 11 1(mod 2), and m t j = Σa ir p jr = 11 1(mod 2) Hence M = [m i j ] is a matrix, such that under modulo 2, 1, f or i = k, t, m i j = 0, otherwise

26 288 Graph Matrices Fig 1011 Example AP T (v 3, v 4 )= In the graph of Figure 1010, we have = v 1 v 2 v 3 v 4 v 5 v (mod2) 108 Adjacency Matrix Let V = (V, E) be a graph with V = {v 1, v 2,, v n }, E = {e 1, e 2,, e m } and without parallel edges The adjacency matrix of G is an n n symmetric binary matrix X = [x i j ] defined over the ring of integers such that 1, i f v i v j E, x i j = 0, otherwise Example Consider the graph G given in Figure 1012 Fig 1012

27 Graph Theory 289 The adjacency matrix of G is given by X = v 1 v 2 v 3 v 4 v 5 v 6 v 1 v 2 v 3 v 4 v 5 v We have the following observations about the adjacency matrix X of a graph G 1 The entries along the principal diagonal of X are all zeros if and only if the graph has no self-loops However, a self-loop at the ith vertex corresponds to x ii = 1 2 If the graph has no self-loops, the degree of a vertex equals the number of ones in the corresponding row or column of X 3 Permutation of rows and the corresponding columns imply reordering the vertices We note that the rows and columns are arranged in the same order Therefore, when two rows are interchanged in X, the corresponding columns are also interchanged Thus two graphs G 1 and G 2 without parallel edges are isomorphic if and only if their adjacency matrices X(G 1 ) and X(G 2 ) are related by X(G 2 ) = R 1 X(G 1 )R, where R is a permutation matrix 4 A graph G is disconnected having components G 1 and G 2 if and only if the adjacency matrix X(G) is partitioned as X(G) = X(G 1) : O : O : X(G 2 ), where X(G 1 ) and X(G 2 ) are respectively the adjacency matrices of the components G 1 and G 2 Obviously, the above partitioning implies that there are no edges between vertices in G 1 and vertices in G 2 5 If any square, symmetric and binary matrix Q of order n is given, then there exists a graph G with n vertices and without parallel edges whose adjacency matrix is Q Definition: An edge sequence is a sequence of edges in which each edge, except the first and the last, has one vertex in common with the edge preceding it and one vertex

28 290 Graph Matrices in common with the edge following it A walk and a path are the examples of an edge sequence An edge can appear more than once in an edge sequence In the graph of Figure 1013, v 1 e 1 v 2 e 2 v 3 e 3 v 4 e 4 v 2 e 2 v 3 e 5 v 5, or e 1 e 2 e 3 e 4 e 2 e 5 is an edge sequence We now have the following result Fig 1013 Theorem 1016 If X = [x i j ] is the adjacency matrix of a simple graph G, then [X k ] i j is the number of different edge sequences of length k between vertices v i and v j Proof We prove the result by using induction on k The result is trivial for k = 0 and 1 Since X 2 = XX, X 2 is a symmetric matrix, as product of symmetric matrices is also symmetric For k = 2, i j, we have [X 2 ] i j = number of ones in the product of ith row and jth column (or jth row) of X = number of positions in which both ith and jth rows of X have ones = number of vertices that are adjacent to both ith and jth vertices = number of different paths of length two between ith and jth vertices Also, [X 2 ] ii = number of ones in the ith row (or column) of X = degree of the corresponding vertex This shows that [X 2 ] i j is the number of different paths and therefore different edge sequences of length 2 between the vertices v i and v j Thus the result is true for k = 2 Assume the result to be true for k, so that [X k ] i j = number of different edge sequences of length k between v i and v j We have, [X k+1 ] i j = [X k X] i j = n [X k ] ir [X] r j = n [X k ] ir x r j r=1 r=1

29 Graph Theory 291 Now, every v i v j edge sequence of length k+1 consists of a v i v r edge sequence of length k, followed by an edge v t v j Since there are [X k ] ir such edge sequences of length k and x r j such edges for each vertex v r, the total number of all v i v j edge sequences of length k+1 is n [X k ] ir x r j This proves the result for k+1 also r=1 We have the following observation about connectedness and adjacency matrix Theorem 1017 Let G be a graph with V = {v 1, v 2,, v n } and let X be the adjacency matrix of G Let Y = [y i j ] be the matrix Y = X + X 2 ++ X n 1 Then G is connected if and only if for all distinct i, j, y i j 0 That is, if and only if Y has no zero entries off the main diagonal Proof We have, y i j = [Y] i j = [X] i j +[X 2 ] i j ++[X n 1 ] i j Since [X k ] i j denotes the number of distinct edge-sequences of length k from v i to v j, y i j = number of different v i v j edge sequence of length 1 + number of different v i v j edge sequences of length 2+ + number of different v i v j edge sequences of length n 1 Therefore, y i j = number of different v i v j edge sequence of length less than n Now let G be connected Then for every pair of distinct i, j there is a path from v i to v j Since G has n vertices, this path passes through atmost n vertices and so has length less than n Thus, y i j 0 for each i, j with i j Conversely, for each distinct pair i, j we have y i j 0 Then from above, there is at least one edge sequence of length less than n from v i to v j This implies that v i is connected to v j Since the distinct pair i, j is chosen arbitrarily, G is connected The next result is useful in determining the distances between different pairs of vertices Theorem 1018 In a connected graph, the distance between two vertices v i and v j is k if and only if k is the smallest integer for which [X k ] i j 0 Proof Let G be a connected graph and let X = [x i j ] be the adjacency matrix of G Let v i and v j be vertices in G such that d(v i, v j ) = k Then the length of the shortest path between v i and v j is k This implies that there are no paths of length 1, 2,, k 1 and so no edge sequences of length 1, 2,, k 1 between v i and v j

30 292 Graph Matrices Therefore, [X] i j = 0, [X 2 ] i j = 0,, [X k 1 ] i j = 0 Hence k is the smallest integer such that [X k ] i j 0 Conversely, suppose that k is the smallest integer such that [X k ] i j 0 Therefore, there are no edge sequences of length 1, 2,, k 1 and in fact no paths of length 1, 2,, k 1 between vertices v i and v j Thus the shortest path between v i and v j is of length k, so that d(v i, v j ) = k Definition: Let G be a graph and let d i be the degree of the vertex v i in G The degree matrix H = [h i j ] of G is defined by 0, f or i j, h i j = d i, f or i = j The following result gives a relation between the matrices F, X and H Theorem 1019 Let F be the modified incidence matrix, X the adjacency matrix and H the degree matrix of a graph G Then FF T = H X Proof We have (i, j)th element of FF T, [FF T ] i j = m r=1 [F] ir [F T ] r j = m r=1 [F] ir [F] jr Now, [F] ir and [F] r j are non-zero if and only if the edge e r = v i v j Then for i j, m 1, i f e r = v i v j is an edge, [F] ir [F] jr = r=1 0, i f e r = v i v j is not an edge For i = j, [F] ir [F] jr = 1 whenever [F] ik = ±1, and this occurs d i times corresponding to the number of edges incident on v i Thus, m [F] ir [F] jr = d i, for i = j r=1 Therefore, 1 or 0, according to whether f or i j, v i v j is an edge or not, [FF T ] i j = d i, f or i = j

32 294 Graph Matrices Therefore, to prove adj Q = τ(g)j, it is enough to prove that τ(g) = any one cofactor of Q Let F 0 be the matrix obtained by dropping the last row from F Then clearly, F o Fo T is a cofactor of Q Using Binet-Cauchy theorem of matrix theory, we have F0 F0 T = F x F T, x (10211) X E where F x is the square submatrix of F o whose n 1 columns correspond to n 1 edges in the subset X of E, the summation running over all possible such subsets We know F x 0 if and only if < X > is a spanning tree of G and Fx = ±1 But, Fx T = F x Therefore each X E such that < X > is a spanning tree of G contributes one to the sum on the right of (10211) and all other contributions are zero Hence F o Fo T = τ(g), proving the theorem Corollary 104 Prove τ(k n ) = n n 2 Proof Q = Here, Q = H X = (n 1)I (J I) = ni J Therefore, = n n n 0 : n n n n 1 1 : n : The cofactor of q 11 is the (n 1) (n 1) determinant given by cofactor of q 11 = n n 1 1 : 1 1 n 1 Subtracting the first row from each of the others and then adding the last n 2 columns to the first, we get

33 Graph Theory 295 cofactor of q 11 = n n 0 : n Expanding with the help of the first column, we have cofactor of q 11 = n n 2 Thus, τ(k n ) = n n Exercises 1 Characterise A f, B f, C f and X of the complete graph of n vertices 2 Characterise simple, self-dual graphs in terms of their cycle and cut-set matrices 3 Show that each diagonal entry in X 3 equals twice the number of triangles passing through the corresponding vertex 4 Characterise the adjacency matrix of a bipartite graph 5 Prove that a graph is bipartite if and only if for all odd k, every diagonal entry of A k is zero 6 Similar to the cycle or cut-set matrix, define a spanning tree matrix for a connected graph, and observe some of its properties 7 If X is the adjacency matrix of a graph G and L is the adjacency matrix of its edge graph L(G) and A and H are the incidence and degree matrices, show that X = AA T H and L = A T A 2I 8 Use the matrix tree theorem to calculate τ(k 4 e) 9 Prove that τ(g) = 1 det(j + Q) n2

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