# A list of $$\|\cdot\|_{\infty \rightarrow 1}^*$$ problems

Recall our old friend the $$\|\cdot\|_{\infty \rightarrow 1}^*$$ norm (from the first version of this website, which I still haven’t gotten around to merging into the current iteration). Given a matrix $$\mat{A},$$ it is a measure of the energy in the smallest expansion of $$\mat{A}$$ in terms of rank one sign matrices:
$\|\mat{A}\|_{\infty \rightarrow 1}^* = \inf\left\{\sum_i d_i \,:\, \mat{A} = \sum_i d_i \vec{u}_i \vec{v}_i, \right\}$
where $$\{\vec{u}_i, \vec{v}_i\}$$ is a collection of sign vectors. Note that the number of sign vectors necessary to achieve this minimum is not known a priori.

This is an interesting norm both intrinsically (as a potential source of factor models in statistics and machine learning) and because of its relations to other norms. See these slides for details.

An estimate of $$\|\cdot\|_{\infty \rightarrow 1}^*$$ to within a multiplicative factor of about $$1.783$$ can be obtained using a semidefinite program, but I suspect it’s NP hard to compute $$\|\cdot\|_{\infty \rightarrow 1}^*$$ exactly (because $$\|\cdot\|_{\infty \rightarrow 1}^*$$ is the trace dual of the $$\|\cdot\|_{\infty \rightarrow 1}$$ norm, which is known to be NP-hard to compute — if the latter is also APX-hard, which I’m not sure of either way, then that means you definitely couldn’t compute $$\|\cdot\|_{\infty \rightarrow 1}^*$$ efficiently in polynomial time). However, more interesting is the question of approximately obtaining the factorization corresponding to $$\|\cdot\|_{\infty \rightarrow 1}^*$$: there doesn’t seem to be a good way to obtain such an approximate factorization from the SDP that gives the approximation of $$\|\cdot\|_{\infty \rightarrow 1}^*$$, but maybe there’s a clever way to do so that I haven’t seen.

Other interesting questions abound: can we characterize $$\|\cdot\|_{\infty \rightarrow 1}^*$$ or the number of rank 1 matrices needed to obtain the corresponding decomposition from a randomly chosen matrix (say i.i.d. Gaussian entries)? What if we just want probability bounds on the number of rank 1 matrices in the decomposition of a randomly chosen matrix?

I think the latter question is probably the easiest to answer of all posed here (except maybe coming up with an SDP-based approximate factorization).

• Stephen

“This is an interesting norm both intrinsically (as a potential source of factor models in statistics and machine learning)”

Is it much more useful than the max-norm? Aren’t they nearly comparable, and the max-norm has the benefit of SDP computation?

• swiftset

Oops. Have not been checking the comments lately. Actually, I made a huge typo in the original post. The norm being considered is the trace dual of the $$\infty \rightarrow 1$$ operator norm, so it can be approximated to within a constant factor using the max-norm. As you say you can then solve for the max-norm with an SDP. The main issue is if you want a factor model, you care about the decomposition itself and not just the value of the norms, and there’s no way I’m aware of to take the decomposition you get from the max-norm SDP and turn it into a sign decomposition of the matrix.

• Jeff

Unrelated, but I can’t seem to find the archive of your old posts before migration to this domain.

There were some interesting problems and posts I like to look at occasionally.