Strongly regular graphs on 306, 400, 576, 784 and 1296 vertices from multi-orbit coset fusion and Bush-type Hadamard matrices
We construct strongly regular graphs with parameters (306,55,4,11), (400,171,74,72), (576,253,112,110), (784,351,158,156) and (1296,595,274,272), and hence also their complements. All five parameter sets were marked "?" (existence unknown) in Brouwer's online table on 10 and 11 October 2026. Three of the graphs were found by a computer search that extends the transitive coset fusion of our earlier papers to groups acting with two orbits: the search fuses the orbitals of a group G on G/H_1⊔ G/H_2, screens with tables of marks, solves the exact quadratic equations on the structure constants…
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