a characteristic b array a; number b b is the generator of the characteristic variety of a. For the algorithm, see Japan J. of Industrial and Applied Math., 1994, 485--497. Example 1 : [ [( (x Dx)^2 + ( y Dy)^2) ( x Dx y Dy -1)] (x,y)] characteristic :: Example 2 : [ [( (x^3-y^2) Dx + 3 x^2) ( (x^3-y^2) Dy - 2 y)] (x,y)] characteristic ::