weird brackets in unit interval I have found during looking at the book Linear Algebra and its Applications (K. Nordstrom) some weird (for me) notation for belonging to the unit interval, namely, $\lambda \in \  ] 0,1 [$. Does it mean as always that $\lambda$ belongs to $[0,1]$ or something different?

The interval $]0,1[$ is the same as $(0,1)$. Further $]0,1]=(0,1]$ and $[0,1[=[0,1)$. It is just an other notation.
If you use the inverse bracket, you can avoid misunderstanding $(0,1)$ as an element of $\mathbb R^2$ instead of the open interval.
