The cohomological Hall algebra conditional surjectivity conjecture

Let XX be a smooth surface, let Z1Z_1 and Z2Z_2 be curves with Z=Z1Z2Z=Z_1\cup Z_2, and let HAX,ZT\mathbf{HA}^T_{X,Z} denote the corresponding TT-equivariant cohomological Hall algebra. Assume that both Z1Z_1 and Z2(Z1Z2)Z_2\setminus(Z_1\cap Z_2) are cohomologically pure. The cohomological Hall algebra conditional surjectivity conjecture. The amalgamation map

HAX,Z2T^HAX,Z1Z2THAX,Z1THAX,ZT\mathbf{HA}^T_{X,Z_2}\widehat{\otimes}_{\mathbf{HA}^T_{X,Z_1\cap Z_2}}\mathbf{HA}^T_{X,Z_1}\longrightarrow\mathbf{HA}^T_{X,Z}

and the induced map from the kernel of the tensor-product map to the kernel of the Hall multiplication map are topologically surjective. The conjecture addresses the failure of ordinary topological surjectivity in general and predicts density under cohomological-purity assumptions. The supplied text gives no resolution evidence, so the claim is recorded as open.

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Primary source

Duiliu-Emanuel Diaconescu, Mauro Porta, Francesco Sala, Olivier Schiffmann and Eric Vasserot, “Nilpotent cohomological Hall algebras of surfaces”, arXiv:2502.19445 (2026).

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