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help gradient

 GRADIENT Approximate gradient.
    [FX,FY] = GRADIENT(F) returns the numerical gradient of the
    matrix F. FX corresponds to dF/dx, the differences in the
    x (column) direction. FY corresponds to dF/dy, the differences
    in the y (row) direction. The spacing between points in each
    direction is assumed to be one. When F is a vector, DF = GRADIENT(F)
    is the 1-D gradient.
 
    [FX,FY] = GRADIENT(F,H), where H is a scalar, uses H as the
    spacing between points in each direction.
 
    [FX,FY] = GRADIENT(F,HX,HY), when F is 2-D, uses the spacing
    specified by HX and HY. HX and HY can either be scalars to specify
    the spacing between coordinates or vectors to specify the
    coordinates of the points.  If HX and HY are vectors, their length
    must match the corresponding dimension of F.
 
    [FX,FY,FZ] = GRADIENT(F), when F is a 3-D array, returns the
    numerical gradient of F. FZ corresponds to dF/dz, the differences
    in the z direction. GRADIENT(F,H), where H is a scalar, 
    uses H as the spacing between points in each direction.
 
    [FX,FY,FZ] = GRADIENT(F,HX,HY,HZ) uses the spacing given by
    HX, HY, HZ. 
 
    [FX,FY,FZ,...] = GRADIENT(F,...) extends similarly when F is N-D
    and must be invoked with N outputs and either 2 or N+1 inputs.
 
    Examples:
        [x,y] = meshgrid(-2:.2:2, -2:.2:2);
        z = x .* exp(-x.^2 - y.^2);
        [px,py] = gradient(z,.2,.2);
        contour(z),hold on, quiver(px,py), hold off