Restriction to finite-index subgroups as \'etale extensions in topology,   KK-theory and geometry
  For equivariant stable homotopy theory, equivariant KK-theory and equivariant derived categories, we show how restriction to a subgroup of finite index yields a finite commutative separable extension, analogous to finite \'etale extensions in algebraic geometry. 
Paul Balmer, Ivo Dell'Ambrogio, Beren Sanders
math.CT math.AG math.AT math.KT