3 papers
math.CO2026
Co-edge-regular graphs with four eigenvalues and unbounded coherent rank
Edwin R. van Dam, Hong-Jun Ge, Jack H. Koolen
In the regular three-eigenvalue setting, spectral complexity and coherent-algebraic complexity coincide: a connected regular graph has exactly three distinct eigenvalues if and onl…
math.CO2026
Almost amorphic association schemes
Edwin van Dam, Jack H. Koolen, Yanzhen Xiong
An association scheme is called amorphic if every possible fusion of relations gives rise to another association scheme. In earlier work, we showed that if an association scheme ha…
math.CO2025
Characterizations of amorphic schemes and fusions of pairs
Edwin R. van Dam, Jack H. Koolen, Yanzhen Xiong
An association scheme is called amorphic if every possible fusion of relations gives rise to a fusion scheme. We call a pair of relations fusing if fusing that pair gives rise to a…