Derived Equivalence in Hyper-Kähler Varieties

• Derived equivalent hyper-Kähler varieties are smooth, compact Kähler manifolds with a unique holomorphic 2-form that yields equivalent derived categories through Hodge and lattice isometries.
• Fourier–Mukai transforms and projectively hyperholomorphic bundles are employed to build explicit kernels, enabling derived equivalences even in twisted setups for K3^[n]-type and generalized Kummer varieties.
• These equivalences establish that Hodge isometries of extended Mukai lattices correspond to isomorphisms in homological and Chow motives, influencing moduli theory and birational geometry.

Derived Equivalent Hyper-Kähler Varieties

A derived equivalent hyper-Kähler variety is a smooth, compact, simply connected Kähler manifold with $H^0(X, \Omega^2_X)$ spanned by a nowhere-degenerate holomorphic 2-form, whose bounded derived category of coherent sheaves is equivalent to that of another such variety. The study of derived equivalence for hyper-Kähler varieties, particularly those of $K3^{[n]}$-type and generalized Kummer type, reveals deep connections between algebraic geometry, Hodge theory, lattice theory, and motives. The D-equivalence conjecture asserts that any two smooth projective birational Calabi–Yau varieties, and specifically birational hyper-Kähler varieties of $K3^{[n]}$-type, are derived equivalent. Recent developments have established this conjecture in key cases and extended its reach to twisted derived categories and motivic invariants (Maulik et al., 2024, Maulik et al., 28 Jan 2026, Yang, 18 Oct 2025).

1. Structural Foundations: Hyper-Kähler Varieties and Their Lattices

A compact hyper-Kähler variety $X$ is irreducibly holomorphic symplectic, simply connected, and satisfies $K_X \cong \mathcal{O}_X$ (Taelman, 2019). For $K3^{[n]}$-type, $X$ is deformation equivalent to a Hilbert scheme $S^{[n]}$ of $n$ points on a K3 surface $S$. The cohomology $H^2(X, \mathbb{Z})$ carries the Beauville–Bogomolov–Fujiki form, yielding canonical isometries to lattices $$\Lambda = H^2(S, \mathbb{Z}) \oplus \mathbb{Z} \cdot \delta$$ with specified intersection properties, notably $\delta^2 = 2-2n$ (Maulik et al., 2024).

Extended Mukai lattices play a pivotal role, rationally and integrally:

$$\widetilde H(X, \mathbb{Q}) = \mathbb{Q} \cdot \alpha \oplus H^2(X, \mathbb{Q}) \oplus \mathbb{Q} \cdot \beta,$$

with $\alpha^2 = \beta^2 = 0$, $(\alpha, \beta) = -1$, and $H^2(X, \mathbb{Q})$ orthogonal to $\alpha,\beta$ (Taelman, 2019, Beckmann, 2021, Yang, 18 Oct 2025). This lattice, equipped with weight-2 Hodge structure and Beauville–Bogomolov pairing, governs the derived category via derived invariance and Torelli-type statements.

2. The D-Equivalence Conjecture and Its Resolutions

The D-equivalence conjecture, formulated by Bondal–Orlov and Kawamata, states that if $X$ and $X'$ are birational projective Calabi–Yau varieties, then $D^b(X) \cong D^b(X')$ as triangulated categories (Maulik et al., 2024). Maulik–Shen–Yin–Zhang prove this for birational hyper-Kähler varieties of $K3^{[n]}$-type by constructing explicit Fourier–Mukai kernels derived from projectively hyperholomorphic bundles (Maulik et al., 2024). These kernels deform along twistor paths in moduli, facilitating derived equivalences across birational models, and extending to the twisted category with arbitrary Brauer classes. The proof leverages wall-crossing, lattice-theoretic decompositions, cohomological criteria for fully faithful functors, and the global Torelli theorem.

For generalized Kummer varieties, the derived equivalence is similarly controlled by an integral “Kum”-type lattice and characterized by Looijenga–Lunts–Verbitsky (LLV) Lie algebra invariance (Yang, 18 Oct 2025, Taelman, 2019).

3. Fourier–Mukai Formalism: Hyperholomorphic and Twisted Kernels

The Fourier–Mukai transform, parametrized by an object $P \in D^b(X \times Y)$, is given by

$$\Phi^P_{X \to Y}(–) = R\pi_{Y*}(P \otimes^L \pi_X^*(–)).$$

Markman's construction produces projectively hyperholomorphic bundles, notably $U[n]$ on $M[n] \times S[n]$, with slope-stability and invariance under hyper-Kähler rotation. These vector bundles deform along twistor paths to yield kernels $E$ of twisted type on $X \times Y$ (Maulik et al., 2024), giving rise to equivalences $D^b(X, \alpha_X) \cong D^b(Y, \alpha_Y)$, where the twists are dictated by explicit Brauer classes computed from Chern data. These functors preserve Mukai pairings, Hodge structure, and, when suitably extended, lead to isomorphisms of homological (and, conjecturally, Chow) motives (Maulik et al., 28 Jan 2026).

Twisted derived categories are naturally indexed by classes $\theta_v \in H^2(X, \mu_{2n-2})$ and their images $[k\theta_v] \in \mathrm{Br}(X)$ (Zhang, 4 Feb 2025). Under parallel-transport Hodge isometries, the equivalences are lifted to the twisted context, extending the Torelli principle to the derived category.

4. Lattice and Motivic Invariants: Derived Torelli Theorems and Algebraicity

Derived equivalence implies, and is often characterized by, existence of Hodge isometries between extended (Mukai or Markman–Mukai) lattices. Taelman proves that the LLV algebra acting on $H^*(X, \mathbb{Q})$ is a derived invariant, enforcing that the extended Mukai lattice must be mapped via a Hodge isometry under any derived equivalence (Taelman, 2019).

For K3 surfaces and $K3^{[n]}$-type varieties, the derived Torelli theorem affirms that $D^b(X) \cong D^b(Y)$ iff their transcendental (or extended) lattices are Hodge isometric. Kapustka–Kapustka extend this classification to twisted categories, depending on Picard rank, divisibility, and Lagrangian fibration structure (Kapustka et al., 2023). For Kum-type varieties, a lattice-theoretic characterization ensures finiteness and Torelli-type results for derived equivalence (Yang, 18 Oct 2025).

Buskin's theorem confirms that any rational Hodge isometry between two $K3^{[n]}$-type varieties is algebraic, i.e., induced by an algebraic cycle—a further compatibility between derived and motivic structure (Maulik et al., 2024).

Recent results (Maulik et al., 28 Jan 2026) establish that derived equivalence for $K3^{[n]}$-type hyper-Kähler varieties implies isomorphism of homological motives, and under Franchetta-type conditions, of Chow motives preserving cup products. Markman's Fourier–Mukai kernels serve to explicitly construct motivic isomorphisms.

5. Applications: Moduli Spaces, Flops, and Twisted Equivalences

The framework applies broadly. Moduli spaces of stable sheaves on K3 surfaces, via suitable Mukai vectors, yield hyper-Kähler varieties or twisted variants. Derived equivalence of these moduli, established by direct use of universal hyperholomorphic bundles, supports conjectures about the structure and motivic invariance of moduli (Kapustka et al., 2023, Maulik et al., 28 Jan 2026).

Wall-crossing phenomena, Mukai flops, and birational transformations among hyper-Kähler varieties induce derived equivalence via explicit kernel constructions. Twisted equivalences are realized by deformations along twistor paths, with precise control over Brauer twists and crystalline data (Maulik et al., 2024, Zhang, 4 Feb 2025).

Lagrangian fibrations and torsors yield higher Picard rank examples, with convolutions of relative Poincaré bundles inducing twisted derived equivalences between distinct varieties of K3$^{[n]}$-type. This methodology generalizes the construction of equivalences well beyond classical cases (Kapustka et al., 2023).

6. Outstanding Problems and Generalizations

Open problems remain: removal of orientation ambiguities in derived monodromy and autoequivalence group characterizations (Taelman, 2019); extension of explicit equivalence constructions to other hyper-Kähler deformation types (OG6, OG10, LLSS, etc.) (Kapustka et al., 2023, Maulik et al., 28 Jan 2026); and a full realization of the derived Torelli theorem for twisted categories absent certain numerical constraints (Zhang, 4 Feb 2025).

On the motivic side, verifying the multiplicative Orlov conjecture and generalizing Chow-motive isomorphism constructions to other hyper-Kähler types and singular/nonprojective settings remains active. The geometry and categorification of the canonical gerbe $\theta_v$ and its functional role in a putative universal K3 category represent promising avenues.

7. Tabular Overview: Classification via Lattices

Variety Type	Classification Lattice	Derived Equivalence Criterion
K3 surface	Mukai lattice $\widetilde H(S, \mathbb{Z})$	Hodge isometry of transcendental
K3$^{[n]}$-type	Markman-Mukai lattice $L(X)$, extended Mukai	Hodge isometry of lattice
Generalized Kummer type	Kum-lattice $\Lambda_X$	Hodge isometry of lattice

For all types, orientation and spinor norm constraints on the Hodge isometry are required, with derived equivalences realized by Fourier–Mukai functors constructed from projectively hyperholomorphic bundles, and classified via their action on these lattices (Maulik et al., 2024, Beckmann, 2021, Yang, 18 Oct 2025).

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The derived equivalence of hyper-Kähler varieties, especially those of $K3^{[n]}$-type, is determined by Hodge-theoretic and lattice-theoretic data, realized concretely via Fourier–Mukai transforms and their motivic consequences. Current research (Maulik et al., 2024, Maulik et al., 28 Jan 2026, Yang, 18 Oct 2025, Kapustka et al., 2023, Taelman, 2019) continues to elucidate the interplay of geometry, category theory, and motives in this context.