  Automorphisms of a perfect complex naturally have the structure of an
$\infty$-group: the 1-morphisms are quasi-isomorphisms, the 2-morphisms are
homotopies, etc. This article starts by proving some basic properties of this
$\infty$-group. We go on to study the deformation theory of this stack of
$\infty$-groups and give a criterion for this stack to be formally smooth. The
classifying stack of this $\infty$-group classifies forms of a complex. We
discuss a version of Hilbert 90 for perfect complexes.

Algebraic Geometry