Optimal control of a robot I have a robot which moves abiding by the following equation:
$$
\frac{d^2 \mathbf{x}}{dt} = -k \frac{d\mathbf{x}}{dt} + \mathbf{a}(t),
$$
where $\mathbf{x}$ is a coordinate vector.
It starts with 
$$
\frac{d\mathbf{x}}{dt} = v_0,$$
and $\mathbf{x} = (x0, y0, z0)$. 
I need to find continuous vector valued acceleration function $\|\mathbf{a}(t)\| \leq c$, which gets robot into spot with coordinates $(0,0,0)$ the quickest  way possible, i.e. as fast as possible. 
I'm not sure how to correctly write my target functional which to minimise.
It seems that my target functional is 
$$
\frac{\partial \int_{t=0}^{t=\tau}\frac{\mathbf{x}(t)}{dt}dt }{\partial \tau} = 0,
$$ 
where $\frac{\mathbf{x}(t)}{dt}$ computed from solution of original differential equation with optimal $\mathbf{a}(t)$.
Because if $\tau$ is optimal, then first derivative with respect to $\tau$ necessary zero. But I'm not sure I've got it right, so I would appreciate a few hints and pointers to the literature.
 A: Then if the constraint concerns the $L^\infty$ norm, that is, $\forall i \in \{x, y, z\} \leq c$, in each individual direction you have a probelm of a starting velocity, a target endpoint, and a maximum acceleration.  You easily solve (for each coordinate) for the time taken to get to that target using the maximal acceleration:  this is a matter of simply solving a quadratic equation, using maximum acceleration in the "right" direction. Of those three arrival times, take the longest; that direction will indeed use maximal acceleration.  Now for the other two coordinates, fix the arrival time at that longest time, and solve for the necessary acceleration to get home at just that instant; it will in each case be less than the maximum allowed.
So the $L^\infty$ case does not have worries about continuity.
If the constraint is n the $L^2$ sense (that is, $a_x^2+a_y^2+a_z^2 \leq c^2$), then the problem becomes more difficult. First, you can always make a homogeneous linear transformation of variables such that you are trying to travel from $(x_0,0,0)$ to the origin, and indeed there is still a rotation about the $X$ axis degree of freedom left, so that the vector $v_0$ can be considered to lie in the $XY$ plane.  These transformations leave the $L^2$ norm constraint unchanged, and turn this into a two-dimensional problem.  
I believe the optimal solution to this two-dimensional problem will use a constant acceleration, although this is not so easy to prove.   The angle of acceleration is dictated by the requirement that both $x$ and $y$ arrive at zero at the same time; there is in general a unique solution.  A bit of algebra arrives at the tangent of the correct angle; again the maximum acceleration magnitude is used, and again there is no continuity worry.
Small perturbations about this solution improve one arrival time, at the cost of making the other later, so this is an extremum. 
Note that your problem did not say you have to end up stopped at the origin. If it did, you would then run up against the "bang-bang" principle.
A: This response is perhaps too late, but... You're looking for the Pontryagin maximum principle applied to minimum time problems. It may be a good idea to look at the extensive set of examples in the canonical and seminal text: Pontryagin, L. S.; Boltyanskii, V. G.; Gamkrelidze, R. V.; Mishchenko, E. F. (1962). The Mathematical Theory of Optimal Processes, Interscience. You need to look for "minimal time" problems in the above text for your answers; the presentation is a bit dated, but the exposition is very detailed and remarkably crisp.
Several later texts are more 'accessible', e.g., the book on Optimal Control by M. Athans and P. Falb. However, be aware that Athans-Falb mostly don't account for the so-called "abnormal multiplier" in their book, except for the proof of the Pontryagin maximum principle; it is important to keep track of the abnormal multiplier at all times, especially in constrained control contexts (such as yours). Modern expositions may be found in D. Liberzon, Calculus of Variations and Optimal Control Theory:
A Concise Introduction; there's a preprint version freely available from the author's webpage here: http://liberzon.csl.illinois.edu/publications.html, and Schättler and Ledzewicz, Geometric Optimal Control, http://www.springer.com/la/book/9781461438335.
For your simple control system, it's possible to give a rather complete description of the optimal controls provided that the manoeuvre is feasible.
