In the paper The Amplituhedron
, Nima Arkani-Hamed and Jaroslav Trnka introduced the geometric object amplituhedron. It is defined as follows (see also the lecture notes).

Let $Z$ be a $(k+m)\times n$ real matrix with maximal minors positive. Let $\tilde{Z}: Gr_{k,n}^{\geq 0} \to Gr_{k,k+m}$ be a map given by $A \mapsto AZ^t$. The tree amplituhedron $\mathcal{A}_{n,k,m}$ is the image $\tilde{Z}(Gr_{k,n}^{\geq 0}) \subset Gr_{k,k+m}$. Here $Gr_{k,n}^{\geq 0}$ is the totally non-negative Grassmannian:
\begin{align}
Gr_{k,n}^{\geq 0} = \{A \in Gr_{k,n}: \Delta_J(A) \geq 0, \forall J \in {[n] \choose k}\},
\end{align}
where $\Delta_J(A)$ is the minor of $A$ using columns $J$.

What is the definition of loop amplituhedrons? Thank you very much.

This post imported from StackExchange MathOverflow at 2017-05-30 20:53 (UTC), posted by SE-user Jianrong Li