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I am considering the following optimization problem. Let $P$ be a set of $n$ points in $\mathbb{R}^d$

maximize $\sum_{p\in P}\vert\langle \Vert p\Vert p, \hat{x}\rangle\vert$ subject to $\Vert\hat{x}\Vert^2=1, \hat{x}\in \mathbb{R}^d$.

Without the absolute value inside the summand, the solution is easy, the optimal solution is achived by the unit vector along the vector $\sum_{p\in P}\Vert p\Vert p$. But with the absolute value in the picture, I failed to find a solution. Can this problem be solved by techniques of optimization literature?

Thanks

p.s.

This problem has some superficial similarities with Chebyshev approximation problem:

$minimize \max_i \vert a_i^Tx-b_i\vert$. The aforementioned problem involves $l_\infty$ norm, whereas the original problem deals with $l_1$ norm.

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  • $\begingroup$ By $||\hat x||$ you mean the $\ell_2$-norm of $\hat x$? $\endgroup$
    – Neal Young
    Dec 26 '20 at 19:24
  • $\begingroup$ Yes. Thank you, I think problem is similar to the problem of Robust Estimator. $\endgroup$ Dec 26 '20 at 19:27
  • $\begingroup$ What's the point of multiplying $p$ by its own norm? Could you not just make $P$ the set of points already scaled the way you want them? $\endgroup$ Mar 5 at 22:51
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The problem as described is not convex, due to the nonconvexity of the constraint set. However, if you were to permit a relaxation, we could write your problem as

$\begin{array}{ll} \max & \sum_{p\in P} t_p\\ \text{subject to }& t_p \geq \langle x, \|p\| p \rangle\\ & t_p \geq -\langle x, \|p\| p\rangle ,\\ & \|x\|_2 \leq 1. \end{array}$

This is a second-order cone program, which can be solved by standard methods.

It is probably not trivial to recover a solution to your original problem from this relaxation (for instance, Goemans-Williamson wrote a breakthrough paper describing the rounding algorithm for a similar relaxation). But maybe this is helpful in some way.

edit -- As pointed out by Neal Young, there is an error in the above answer. The question in the original post is a maximization of absolute value function, which isn't a convex objective; so, my reduction is incorrect. I am leaving the incorrect answer here in case someone finds any information in it useful, but please see Neal Young's comments below for more comprehensive (and correct) observations about the problem.

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    $\begingroup$ Your proposed problem has constraints "$t_p \ge \cdots$" (two times), and objective $\max \sum_p t_p$, so the problem is unbounded (take $t_p$ arbitrarily large). You could try "$t_p \le \cdots$", but then you would have $t_p = -|\langle x, \|p\|p\rangle|$ and taking $x = \overline 0$ would be optimal.. $\endgroup$
    – Neal Young
    Mar 1 at 21:41
  • $\begingroup$ @NealYoung Thank you for pointing out that error! I am just realizing that the problem stated in the original post is itself nonconvex, since it asks about maximizing a sum of absolute values. So perhaps my approach is totally wrong, since it would be too much to expect to have a convex program so close to a nonconvex problem. $\endgroup$ Mar 1 at 22:02
  • $\begingroup$ @NealYoung Should I just delete my answer? I am new, so I am not sure what to do about wrong answers I write. $\endgroup$ Mar 1 at 22:02
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    $\begingroup$ I'm not sure if there is a standard practice for this. Sometimes people leave the answer, but edit it to add a comment to the top noting that it's wrong. I guess people do that if they think the answer, though wrong, still contains information that other readers will find useful. $\endgroup$
    – Neal Young
    Mar 2 at 16:23

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