x  y  dx  dy
2.   x  y  dx (dy)
3.   x  y (dx) dy
4.   x (y)(dx) dy
5.  (x) y  dx (dy)
6.  (x)(y) dx  dy

By inspection, it is fairly easy to understand Df
as telling you what you have to do from each point
of U in order to change the value borne by f(x, y).

Jon Awbrey

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<A  HREF="http://www.cs.bsu.edu/homepages/mighty/history.html">http://www.cs.bsu.edu/homepages/mighty/history.html</A>
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