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Question: the problem about sum

Question:the problem about sum

yerui407 0 Maple
student
June 03 2010
0

> restart;
> with(linalg);
> uvec := array(1 .. 2, [u_(x, y, t), v_(x, y, t)]):
> xvec := array(1 .. 2, [x, y]):
> equns := array(1 .. 3, []):
> for i to 2 do

   equns[i] := sum('uvec[j]*(diff(uvec[i], xvec[j]))', 'j' = 1 .. 2)

   end do;

the result the maple 11 gives is:
                                      equns[1] := 0
                                      equns[2] := 0

however,the correct result should be:

                                     equns[1] := u_(x, y, t)*(diff(u_(x, y, t), x))+v_(x, y, t)*(diff(u_(x, y, t), y))

                                      equns[2] := u_(x, y, t)*(diff(v_(x, y, t), x))+v_(x, y, t)*(diff(v_(x, y, t), y))

It makes me confuse.why?
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