Non-commutative varieties with curvature having bounded signature
  The signature(s) of the curvature of the zero set V of a free (non-commutative) polynomial is defined as the number of positive and negative eigenvalues of the non-commutative second fundamental form on V determined by p. With some natural hypotheses, the degree of p is bounded in terms of the signature. In particular, if one of the signatures is zero, then the degree of p is at most two. 
Harry Dym, J. William Helton, and Scott McCullough
math.FA