Reflection-vector conjecture for exceptional objects and collections

Let XX be the variety under consideration, let Db(X)D^b(X) be its bounded derived category, and let Ψq\Psi_q assign to an object of Db(X)D^b(X) its quantum-cohomological vector. Assume that the quantum cohomology of XX has reflection vectors, denoted by R\mathcal{R}, with pairing (xy)(x|y) and monodromy group WW. An object of Db(X)D^b(X) is exceptional if

Extk(E,E)={Cif k=0,0otherwise,\operatorname{Ext}^k(\mathcal{E},\mathcal{E})=\begin{cases}\mathbb{C}&\text{if }k=0,\\0&\text{otherwise},\end{cases}

and a sequence of exceptional objects is a full exceptional collection if it generates Db(X)D^b(X) under isomorphisms, shifts, and cones.

Reflection-vector conjecture. (a) If the quantum cohomology of XX is convergent and semisimple, then Ψq(E)\Psi_q(\mathcal{E}) is a reflection vector for every exceptional object EDb(X)\mathcal{E}\in D^b(X). (b) If (E1,,EN)(\mathcal{E}_1,\dots,\mathcal{E}_N) is a full exceptional collection in Db(X)D^b(X), then the reflection vectors αi:=Ψq(Ei)\alpha_i:=\Psi_q(\mathcal{E}_i) (1iN)(1\leq i\leq N) generate the set R\mathcal{R} of all reflection vectors in the following sense: the reflections xx(xαi)αix\mapsto x-(x|\alpha_i)\alpha_i (1iN)(1\leq i\leq N) generate the monodromy group WW of quantum cohomology, and for every αR\alpha\in\mathcal{R} there exists wWw\in W such that w(α){α1,,αN}w(\alpha)\in\{\alpha_1,\dots,\alpha_N\}.

This relates convergence and semisimplicity of quantum cohomology to exceptional objects and full exceptional collections in the derived category. The supplied text does not state whether the conjecture has been proved or remains open.

Sources & referencesView supporting material

Primary source

Todor Milanov and Xiaokun Xia, “Reflection Vectors and Quantum Cohomology of Blowups”, arXiv:2304.04365 (2024).

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