## Wednesday, May 2, 2012

### Optimal Shoelacing

There are gazillion alternative ways to tie your shoelaces of which only a few patterns are easy to remember and possess nice symmetry.

My daughter got her first laced shoes a few weeks ago. As she took it out of the box, the first noticeable thing was that the slack required to tie the laces appeared to be on the shorter side. This posed difficulties for my daughter while she was learning to make those neat knots. Rewiring the shoes resulted in quite a bit of slack, which resulted in big knots that not only helped her learn quickly, but also seemed irresistible to her kindergarten classmates who kept tugging at it. So the aim was to find a lacing pattern that would generate just the right amount of slack.

This is of course an instance of a Traveling Salesman Problem (TSP). Each lace hole represents a city which can be visited exactly once. For example, the slack is maximized by finding the shortest tour, while the slack can be minimized by finding the longest Hamiltonian circuit. Of course, my daughter would prefer finding a good balance between these two extreme solutions, while also ensuring that tightening and loosening the laces are relatively easy to perform. The former objective can be equivalently specified in terms of minimizing deviation from a desired tour length, while the latter requirement can perhaps be approximated by eliminating unfavorable connection patterns and reducing overall friction.

Any OR person will tell you this: just because the TSP is a notorious NP-Hard problem, it does not automatically mean that practical instances are terribly difficult to manage. On the contrary, OR methods excel in quickly finding amazingly (and provably) good answers to practical instances of underlying TSP substructures within decision problems across a variety of industries.