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math.CO2026
Graphs with three eigenvalues from group divisible designs
Gary Greaves, Jose Yip
We construct biregular graphs with three distinct eigenvalues from semiregular group divisible designs. For every prime power , we use the spectral structure of Muzychuk's Const…
math.CO2024
The coherent rank of a graph with three eigenvalues
Gary Greaves, Jose Yip
We characterise graphs that have three distinct eigenvalues and coherent ranks 8 and 9, linking the former to certain symmetric 2-designs and the latter to specific quasi-symmetric…