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Inmathematics, ingraph theory, theSeidel adjacency matrix of asimple undirected graphG is asymmetric matrix with a row and column for each vertex, having 0 on the diagonal, −1 for positions whose rows and columns correspond to adjacent vertices, and +1 for positions corresponding to non-adjacent vertices.It is also called theSeidel matrix or – its original name – the (−1,1,0)-adjacency matrix. It can be interpreted as the result of subtracting theadjacency matrix ofG from the adjacency matrix of thecomplement ofG.
Themultiset ofeigenvalues of this matrix is called theSeidel spectrum.
The Seidel matrix was introduced byJ. H. van Lint andJohan Jacob Seidel [de;nl] in 1966 and extensively exploited by Seidel and coauthors.
The Seidel matrix ofG is also the adjacency matrix of asigned complete graphKG in which the edges ofG are negative and the edges not inG are positive. It is also the adjacency matrix of thetwo-graph associated withG andKG.
The eigenvalue properties of the Seidel matrix are valuable in the study ofstrongly regular graphs.
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