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An elliptic operator $L$ is called uniformly elliptic if $a^{ij}(x)\xi _i\xi_j \ge \theta|\xi|^2$. What does this notation mean? All I can find about it is that it is some kind of summation notation or something. Can anyone explain me what it is?

Thank you very much.

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    Satisfied with the answer below?2012-07-15
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    @did : understood the definition but i am not able to appreciate ,why it should be defined that way ?2012-07-15
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    Well, for one thing, for the LHS to be a number, starting from a vector $(\xi_i)_i$ and a square matrix $(a_{ij}(x))_{ij}$. In the end, I do not know why you leave the question open.2012-07-15
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    @did : Sir , i didn't understand what u meant by leaving the question open .2012-07-15

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