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This is just a quick check: Is the derivative of the function $f(\vec{x})=\vec{x}$ equal to $1$ or the identity matrix? (I know that this question is quite stupid, but I want to get my definitions right...) Thank you.

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up vote 3 down vote accepted

If "derivative" means "Jacobi matrix", then it is the identity matrix.

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Thanks, Rasmus. Is there more than one definition to "derivative" of that function? – Pete Nov 22 '11 at 9:41
@Pete: You might want to study this wikipedia article. – Rasmus Nov 22 '11 at 17:32

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