On the variety of triangles for a hyper-Kaehler fourfold constructed by Debarre and Voisin

Abstract: We study the similarities between the Fano varieties of lines on a cubic fourfold, a hyper-Kaehler fourfold studied by Beauville and Donagi, and the hyper-Kaehler fourfold constructed by Debarre and Voisin. We exhibit an analog of the notion of "triangle" for these varieties and prove that the 6-dimensional variety of "triangles" is a Lagrangian subvariety in the cube of the constructed hyper-Kaehler fourfold.