Explicit differential characterization of PDE systems pointwise   equivalent to Y_{X^{j_1}X^{j_2}}=0, 1\leq j_1,j_2\leq n\geq 2
  In this paper, a direct continuation of math.DG/0411165, we generalize S. Lie's linearization criterion of an ordinary second order differential equation to the case of several independent variables (x^1, x^2 ..., x^n), n >1, and a single dependent variable y. Strikingly, as in math.DG/0411165, the (complicated) characterizing differential system is of first order. By means of computer programming, this phenomenon was discovered in the case n=2 by S. Neut and M. Petitot (www.lifl.fr/~neut/recherche/these.pdf). 
Joel Merker