diff is not the problem here, subs is. For example, if you compare the
expressions for diff(x, y) and diff(y, x), they are the same. The problem is
that you get terms like (2*x - 3*y**2)/(x**2 + y**2). IMHO, subs({x:0, y:0})
should not even work here, because you get 0/0. The problem is that
Hi,
I know this problem can be treated correctly as above. The example
given here is just to say that using "diff" has to be more careful
since
diff(f,x,y).subs([(x, a), (y,b)])
may be not equal to $lim_{(x,y)\to(a,b)} f(x,y)$.
Thanks,
cch
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