# Archive | flotsam

## Fact of the day: Functions with given merge tree

$$\def\Real{\mathbb{R}}$$

Consider a Morse function on $$f:\Real^n\to \Real$$ with controlled behavior at infinity, – say, $$f=|x|^2$$ near infinity. Assume further that all critical values $$a_1<a_2<\ldots<a_k$$ are distinct and that all indices of critical points are $$0$$ or $$1$$. (Condition obviously holds in one variable.)

Clearly, there are many function that satisfy these conditions. In the (again, most transparent) univariate case, the enumeration of topological types of functions with given critical values is the subject of a nice thread of papers by Arnold on “snakes” (see, e.g., here), relating them to enumeration of ramified coverings of Riemannian spheres, up-down sequences and other cute objects.…

## Mapping department strength

What is your math department strength? One of those rather polarizing questions, or an opportunity to gossip without guilt.

Still, important to understand where your department is on the map, if one wants to steer it in some particular direction, or to keep it where it is.

I wrote a short python script, using BeautifulSoup (thank you!) and campus-wide subscription to MathSciNet (thank you too!) – and downloaded the raw numbers: how many items (whichever MathSciNet is indexing: articles, books, theses,…) are authored by folks from ‘1-IL’ (our code) vs.…

## Shuffling the sheep

$$\def\xx{\mathbf{x}}\def\Real{\mathbb{R}}$$

We consider the problem of flock control: what are the natural constraints on the steering agents in reconfiguring an ensemble of agents?

Our basic setup is following: the agents are modeled as points $$\xx=(x_1,\ldots,x_n), x_k\in X, k=1,\ldots, N$$, with $$X$$ some terrain (for simplicity, we constrain ourselves here to Euclidean plane, but in general it can be some reasonably tame subset of a manifold). Quasi-static behavior is assumed, under which the system, for each value of the controlling parameters quickly settles into (locally minimizing) the equilibrium whose basin of attraction contains current position.…

## Protected: Curvilinear Origami

There is no excerpt because this is a protected post.

## Fact of the day: Brownian centroids

$$\def\Real{\mathbb{R}}\def\Z{\mathcal{Z}}\def\sg{\mathfrak{S}}\def\B{\mathbf{B}}\def\xx{\mathbf{x}}\def\ex{\mathbb{E}}$$

Consider $$n$$ points in Euclidean space, $$\xx=\{x_1,\ldots, x_n\}, x_k\in \Real^d, n\leq d+1$$.

Generically, there is a unique sphere in the affine space spanned by those points, containing all of them. This centroid (which we will denote as $$o(x_1,\ldots,x_n)$$) lies at the intersection of the bisectors $$H_{kl}, 1\leq k\lt l\leq n$$, hyperplanes of points equidistant from $$x_k, x_l$$.

Assume now that the points $$x_k,k=1,\ldots,n$$ are independent standard $$d$$-dimensional Brownian motions. What is the law of the centroid?

It quite easy to see that it is a continuous martingale.…

## Fact of the day: Condorset domains, tiling permutations and contractibility

$$\def\Real{\mathbb{R}}\def\Z{\mathcal{Z}}\def\sg{\mathfrak{S}}\def\B{\mathbf{B}}$$

Consider a collection of vectors $$e_1,\ldots,e_n$$ in the upper half-plane, such that $$e_k=(x_k,1)$$ and $$x_1>x_2\gt \ldots \gt x_n$$. Minkowski sum of the segments $$s_k:=[0,e_k]$$ is a zonotope $$\Z$$. Rhombus in this context are the Minkowski sums $$\Z(k,l)=s_k\oplus s_l, 1\leq k\lt l\leq n$$ of pairs of the segments. Tilings of $$\Z$$ by $$n\choose 2$$ rhombuses $$\Z(k,l)$$ define also an oriented graph (formed by the edges of the rhombuses, and oriented up in the plane). An increasing path from the bottom to the top defines then a permutations in $$\sg_n$$: each of the vectors $$e_k$$ appears in an increasing path once, and the permutation then is the order of these vectors.…

## Topologically constrained models of statistical physics.

$$\def\Real{\mathbb{R}} \def\Int{\mathbb{Z}} \def\Comp{\mathbb{C}} \def\Rat{\mathbb{Q}} \def\Field{\mathbb{F}} \def\Fun{\mathbf{Fun}} \def\e{\mathbf{e}} \def\f{\mathbf{f}} \def\bv{\mathbf{v}} \def\blob{\mathcal{B}}$$

### The Blob

Consider the following planar “spin model”: the state of the system is a function from $$\Int^2$$ into $$\{0,1\}$$ (on and off states). We interpret the site $$(i,j), i,j\in\Int$$ as the plaque, i.e. the (closed) square given by the inequalities $$s_{ij}:=i-1/2\leq x\leq i+1/2; j-1/2\leq y\leq j+1/2$$.

To any configuration $$\eta$$ we associate the corresponding active domain,
$A_\eta=\bigcup_{(i,j): \eta(i,j)=1} s_{ij}.$

We are interested in the statistical ensembles supported by the finite contractible active domains – let’s refer to such domains as blobs.…

## Topology in Motion @ICERM Fall’16

If you are interested in Applied Topology, tend to plan far, far ahead and have some free time on your hands in Fall 2016, check this out, and let us know!