 Witten spinor equation

This elliptic equation on a spacelike spin hypersurface uses the restriction of the spacetime spin connection, including extrinsic curvature. The spinor tends to a prescribed constant at spatial infinity. It removes the negative derivative square in the divergence of the Nester two-form. It is weaker than the spacetime Killing spinor equation; generic positive-energy initial data need not admit any Killing spinor.