Kerr Initial data
  Exploiting a 3+1 analysis of the Mars-Simon tensor, conditions on a vacuum initial data set ensuring that its development is isometric to a subset of the Kerr spacetime are found. These conditions are expressed in terms of the vanishing of a positive scalar function defined on the initial data hypersurface. Applications of this result are discussed. 
Alfonso Garc\'ia-Parrado G\'omez-Lobo and Juan A. Valiente Kroon