How does the velocity of a [tachyon](https://en.wikipedia.org/wiki/Tachyon) transform under a Lorentz boost? Suppose we only consider motion along the $x$ direction for simplicity. If the velocity of the tachyon is $u$ in the lab frame, what is the velocity $u'$ of the tachyon in a frame moving with velocity $v$ (slower than $c$) relative to the lab frame? Can we still use the formula

$$
u' = \frac{u-v}{1-\frac{uv}{c^2}}
$$