Given some $n ∈ ℤ$ what conditions must $v$ satisfy for $n \left \lfloor {v} \right \rfloor $ = $\left \lfloor {n v} \right \rfloor $ I'm probably overthinking this.
What constraints must you place on $v\in \mathbb R$ : $n \left \lfloor {v} \right \rfloor $ = $\left \lfloor {n v} \right \rfloor $ if $n$ is an arbitrary integer?
I can tell that $v \in ℤ$ works eg. $2 \left \lfloor {3} \right \rfloor = \left \lfloor {2 \times 3} \right \rfloor, $ but I'm wondering if I'm missing a more subtle set of constraints on $v$.
Any help (even instructive comments/hints) would go a long way.
Thanks for your time.
 A: This is for $n>0$ and $v>0$.
Write $v=x+y$, where $x$ is an integer and $0\le y<1$, so $\lfloor v\rfloor=x$. Next, 
$$
\lfloor nv\rfloor=\lfloor nx+ny\rfloor=nx+\lfloor ny\rfloor.
$$
Can you do the rest?
A: Here is a powerful result (stronger than needed, though), which can be used to deal with this problem.  From the theorem below, you would see that $\{x\}\in\left[0,\dfrac1n\right)$ for $x\in\mathbb{R}$ to satisfy $$\lfloor nx\rfloor=n\,\lfloor x\rfloor\,,$$ where $n\in\mathbb{Z}_{>0}$ is fixed.  Here, $\{x\}$ is the fractional part of a real number $x$, 

Theorem. For each $x\in\mathbb{R}$ and $n\in\mathbb{Z}_{>0}$,
  $$\lfloor nx\rfloor=\sum_{k=0}^{n-1}\,\left\lfloor x+\frac{k}{n}\right\rfloor\text{ and }\lceil nx\rceil=\sum_{k=0}^{n-1}\,\left\lceil x-\frac{k}{n}\right\rceil\,.$$ 

For $n\in\mathbb{Z}_{<0}$, use the fact that $\lfloor x\rfloor =-\big\lceil (-x)\big\rceil$ for all $x\in\mathbb{R}$.  The theorem above shows that $$\{x\}\in \{0\}\cup\left(1+\frac{1}{n},1\right)$$ if $\lfloor nx\rfloor=n\,\lfloor x\rfloor$ for a given $n\in\mathbb{Z}_{<0}$.  For $n=0$, any $x$ works.
