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 notation $]a,b[$ is used for an open interval, more commonly written as $(a,b)$; meaning:
$$x \in \; ]a,b[  \; \iff a \color{red}{<} x \color{red}{<} b$$
whereas:
$$x \in [a,b] \iff a \color{blue}{\le} x \color{blue}{\le} b$$
So $\lambda \in \; ]0,1[$ would mean values satisfying $0<\lambda<1$, excluding the end points of the interval.
Half-open intervals are then written in a similar way, e.g. $[a,b) = [a,b[$ etc.
This notation is more common in the French school (and countries adopting that notation) and has the advantage of avoiding confusion since $(a,b)$ is a common notation with other meanings too.
