Monthly Archives: February 2019

Median Shapes

When I wrote the paper with Simon Morgan pointing out the L^1\text{TV} functional was actually computing the flat norm for boundaries, we suggested this gave us a computational route to statistics in spaces of shapes. While earlier work certainly touched on this idea of using the flat norm for inference in shape spaces — see this paper on shape recognition, it was not until my student Yunfeng Hu collaborated with myself and Bala Krishnamoorthy (my collaborator, also a co-mentor of Yunfeng’s), that we started addressing the idea of statistics in shape spaces in the original paper with Simon Morgan.

The results can be found here: https://arxiv.org/abs/1802.04968 , in a paper with the title Median Shapes, with authors Yunfeng Hu, Matthew Hudelson, Bala Krishnamoorthy, Altansuren Tumurbaatar, Kevin R. Vixie. Tumurbaatar wrote the first complete version of the code used, and Matthew Hudelson contributed a pivotal new result on graphs inspired by a problem in the paper, while Bala led the computational end of things and I led, in collaboration with Yunfeng Hu (and Bala keeping us honest!), the theoretical parts of the paper. It was a fair bit of work.

We went over the more difficult results a few times, finding improvements and corrections. Of course, there may be a few things here and there to improve, but for now, it is done.

Yunfeng probably spent the most time writing up the piece proving that near regular points on the median, the collection of minimal surfaces meeting the median have a tangent structure we describe as a book.  While this is clear to experienced geometric analysts,  there are lots of little details and we wanted most of the paper to be more accessible to a wider audience. There are lots of other pieces here and there that took time to think about and write up (and rewrite). For example, when showing the set of medians need not contain any regular members,  the part where we show that we need only consider graphs when searching for a minimizer was not easy. And of course, as in most all of geometric analysis, there are problems you solve without too much effort at a high level, but find that writing down is tedious, though at times enlightening due to the fact that those little details turn out to be hard and illuminating.

Because the problem of computing the median reduces to a linear program, while the mean reduces to a quadratic program, we focused on the median problem. Some parts of the paper are a bit long winded, for the reason that we wanted it to have more details that would usually be in a paper communicating to others that understand geometric analysis.

Anyway, have a look. If you find yourself interested, there is already code you can use to compute medians, though we hope eventually to have faster code.

 

A Silence that is Rich in Inspiration

Glynne Robinson Betts’ 1981 Writers in Residence is for me a lyrical invitation to quietness. The photo illuminated essays on places, in time and space, where writers lived and wrote,  invoke a strong sense of life lived with wide spaces for thought and creativity. The sections on Carl Sandburg, Anne Dillard, Robinson Jeffers and several others, recall daily rhythms friendly to depth.

These passages somehow bring back my life in the late 60’s and 70’s, when computers were rare and time to think was not hard to find. In college in the early 80’s, there was still time to think, time to read through a book on a weekend or master complex ideas in quietness, with a devotion to deep comprehension. I wonder, how many now feel that call to stillness, to a silence that is rich in inspiration?

Where do we find the wide spaces today? And who lives there? Those that tasted the thrill of illumination through immersion in quietness remember those spaces, but what of those addicted to their mobile devices, what of those who believe social media connects, wikipedia illuminates and TED talks are the pinnacle of inspiration? While wikipedia is useful, TED talks are sometimes narrowly inspiring, the intense illumination of bare-handed, personal discovery leaves you changed, forever.

Seeking the simplicity of those wide spaces, listening till we hear the quietness sing, we find the same places the writers found, the same illumination that is never forgotten … we walk through an open door to the infinity that lives between the ticks of the clock, between the words on a page, between the breaths we breath.