Markman--Markushevich principle and Lagrangian fibrations on O'Grady six-manifolds

Abstract: To any compact holomorphically symplectic manifold and a nef BBF-isotropic class we associate a quasi-projective variety called the Markman--Markushevich space which parametrises the Lagrangian fibrations with this isotropic class modulo Tate--Shafarevich twists. We give a systematic interpretation of known results about Tate--Shafarevich twists of Lagrangian fibrations using this space, employ it to obtain a conjectural result for the last deformation type for which such a result is not yet proposed, and make some first steps towards its proof. As a byproduct, we generalise the results of Kumar on existence Shioda--Inose partners for abelian surfaces to a slightly more general class of K3 surfaces, and associate a canonical K3 surface with transcendental lattice isometric to $U<sup>2\oplus\langle-2\rangle<sup>2$ to a smooth cubic surface with a choice of a Schläfli double-six. We also review some other approaches towards the construction of complete families of O'Grady six-manifolds with Lagrangian fibrations, and show how they may still be useful even though they do not quite work.