#
Positivity, graphical models, and modeling of complex multivariate dependencies

October 13 to October 17, 2014
at the

American Institute of Mathematics,
Palo Alto, California

organized by

Dominique Guillot,
Apoorva Khare,
Helene Massam,
and Bala Rajaratnam

## Original Announcement

This workshop will be devoted to studying
functions that preserve Loewner properties on (distinguished submanifolds of)
the cone of positive semidefinite matrices.
Understanding complex multivariate dependencies and structure in very high
dimensions is a fundamental challenge of the 21st century, and has applications
in a variety of scientific disciplines. Covariance/correlation matrices are the
most common measures of dependency used in such applications.
In order to obtain sparse covariance estimators a recent approach that has shown
much promise consists of applying entrywise functions to the entries of sample
correlation matrices. This approach is immediately scalable to ultra
high-dimensional settings. However, for this technique to be useful, it is
important that the resulting matrices be positive definite.

The above problem turns out to be one of analysis on the cone, as compared to
$L^1$-based optimization on the cone, with the latter being the standard approach
to such problems. Analysis on the cone has a rich history and has resurfaced
because of modern day applications. From a classical viewpoint, various notions
of preserving positivity by applying functions (e.g., entrywise or via the
Hermitian functional calculus) have been studied in the past, starting from the
work of Schoenberg and Rudin, where the focus is on matrices of all dimensions.
On the other hand, in applications one works only in fixed dimension. However,
characterizing the functions preserving positivity in fixed dimension is only
known in the $2 \times 2$ case and is a considerably harder problem. Thus, the broad
mathematical question which the workshop will tackle, is to characterize the
entrywise functions which preserve Loewner positivity, on positive semidefinite
matrices of fixed dimension and on sub-families of matrices with specific
constraints. Under the aforementioned broad framework, we have identified, among
others, specific areas of focus.

- Entrywise functions preserving Loewner positivity in a given fixed dimension
(since in applications dimension of problems are known).
- Preserving positivity under rank constraints (as small samples sizes lead to
sample covariance matrices of low rank).
- Preserving positivity under sparsity constraints (to induce sparse graphical
models).

## Material from the workshop

A list of participants.
The workshop schedule.

A report on the workshop activities.

Papers arising from the workshop: