Yuhan Zhang

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math.CO2026

Dense-core approach to the Brualdi--Hoffman--Turán problem on odd wheels

Longfei Fang, Mingqing Zhai, Yuhan Zhang

We present a unified presentation of the fixed-size adjacency-spectral extremal problem for odd wheels $W_{2 k + 1}$ , where $k \geq 2$ and $W_{2 k + 1} = K_{1} \lor C_{2 k}$ . The exceptional cas…

math.CO2026

A spectral threshold for triangle counting

Yuhan Zhang, Mingqing Zhai

The 1970 spectral extension of Mantel's theorem, proved by Nosal, states that every graph with $m$ edges and spectral radius $ρ_{1} > m$ contains at least one triangle. Its quan…