Strongly Regular Graphs with Maximal Energy

The energy of a graph is the sum of the absolute values of the eigenvalues of its adjacency matrix. Koolen and Moulton have proved that the energy of a graph on n vertices is at most n(1+?n)/2, and that equality holds if and only if the graph is strongly regular with parameters (n, (n+?n)/2, (n+2?n)/4, (n+2?n)/4). Such graphs are equivalent to a certain type of Hadamard matrices. Here we survey constructions of these Hadamard matrices and the related strongly regular graphs.