The set of rational numbers in [0,1]

yifli · 2011-06-06T17:31:36+00:00

this set is neither closed nor open, correct? the boundary of this set is the closed interval [0,1] because every ball centered at 0<=t<=1 contains both rational numbers and irrational numbers, am I right?

Thread 'How does time derivative commute from one variable to another?'

I'm reviewing Meirovitch's "Methods of Analytical Dynamics," and I don't understand the commutation of the derivative from r to dr: $$ \mathbf{F} \cdot d\mathbf{r} = m \ddot{\mathbf{r}} \cdot d\mathbf{r} = m\mathbf{\dot{r}} \cdot d\mathbf{\dot{r}} $$

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Thread 'How does time derivative commute from one variable to another?'

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