## An Intriguing Tapestry: Number triangles, polytopes, Grassmannians, and scattering amplitudes

(This is a duplicate of a Mathoverflow question posed in Oct 2014 that ran the gauntlet of the OCD cadre there–the demonstrative ones I assume avoid stepping on cracks in the sidewalk and become obstructive, hostile, and/or jealous at popular questions they can’t immediately circumscibe within their own rigid, limited views on how and what math should be done, naturally incuding terminology: Bernoullians!? Blasphemy! See also this blog post and this MO-Q. In a couple of days the question got around 800 views, 9 upvotes, 7 downvotes, two or three close requests, several unconstructive/petty comments on rewriting/formatting, but no answers. Recognizing, from experience on MO, the futility of further refining the question and having pursued a plausible partial answer myself, I deleted most of it. Here is the full text so that I and others can easily follow related references.)

Last month (Sept. 2014) the workshop New Geometric Structures in Scattering Amplitudes was hosted by CMI with the following partial overview:

Recently, remarkable mathematical structures have emerged in the study of scattering amplitudes, revealing deep links to algebraic geometry, arithmetic and combinatorics. There have been many exciting dual representations of amplitudes including

•  polytopes, twistor diagrams and the positive grassmannian leading to the concept of the amplituhedron,
•  polylogs, multizeta values and multiloop integrals,
•  …

Each of these disparate ideas and methods have their own remarkable successes, and indeed have led to important progress in related areas of mathematics. They also face obstacles that they need to overcome in order to become important tools in resolving the most important open conjectures in the field and for the explicit construction of amplitudes.

Classic Number Clans in the Tapestry

It’s a rich tapestry in the making (note this Dec. workshop), weaving together much interesting math and physics, in which I would like to follow some threads particularly intriguing to me (since they relate to different perspectives on the combinatorics of polytopes and simplicial complexes related to Lagrange compositional inversion).

The two bullets above relate to two threads

I ) $T_V$: Totally non-negative Grassmannians $G^+(k,n)$ ~ matroid polytopes $P_M$ ~ $Vol(P_M)$ ~ degrees of toric varieties ~ number of solutions of polynomial equations related to scattering amplitudes in twistor string theory from volume/contour integrals over $G^+$.

II) $T_C$: $G^+$ ~ Stasheff associahedra ~ cluster algebras/coordinates ~ generalized polylogarithms ~ MZVs ~ multiloop integrals for scattering amplitudes

It was Marni Sheppeard through her paper “Constructive Motives and Scattering]” who first gave me a docent’s tour of this tapestry. In particular, she points out some threads interweaving Grassmannians, associahedra, cluster algebra, generalized permutohedra, volumes of hypersimplices and the Eulerian numbers, volumes (and binary trees) enumerated by the Narayana numbers, and scattering amplitudes, among others.

And, Lauren Williams in “Enumeration of totally positive Grassmann cells]” develops a polynomial generating function $A_{k,n}(q)$ whose $q^d$ coefficient is the number of totally positive cells in $G^+(k,n)$ that have dimension $d$ and goes on to show that for the binomial transform $\hat{E}_{k,n}(q)=q^{k-n}\sum^n_{i=0} (-1)^i \binom{n}{i} A_{k,n-i}(q)$ that $\hat{E}_{k,n(}(1)=E_{k,n}$, the Eulerians, and $\hat{E}_{k,n}(0)=N_{k,n}$, the Narayanaians. She reiterates this in her presentation “The Positive Grassmannian (a mathematician’s perspective)” and relates G+ to soliton shallow-water-wave solutions of a KP equation, noting the roles of $G^+$ in computing scattering amplitudes in string theory, a relation to free probability, and the occurrence of the Eulerians and Narayanaians in the BCFW recurrence and twistor string theory.

The number clans that appear in the tapestry are listed below along with some associations. (The Wardians seem peripheral for the moment, but they do lead to the associahedra through fans and phylogenetic trees, and the refined ones can be scaled to the refined f-vectors of the associahedra through their relation to Lagrange inversion.)

Some relations to number clans:

Eulerians, $E_{n,k}$ (A008292, refined-A145271):

h-vectors of simplicial complexes dual to the permutohedra ~ volumes of hypersimplices ~ degrees of varieties ~ number of solutions of polynomials for scattering amplitudes

Catalanians, $C_{n,k}$ (A033282, refined-A133437):

f-vectors of Stasheff associahedra (for Coxeter group $A_n$), related to dissections of polygons (with the Catalans, # of vertices, enumerating the triangulations) ~ structure of associahedra reflects cluster algebra relations

Narayanaians, $N_{n,k}$ (A001263, refined-A134264):

h-vectors of the simplicial complex dual to the Stasheff associahedra, sum to the Catalan numbers, enumerate non-crossing partitions on [n] and refinement of binary trees (right-pointing leaves), refined Narayanaians relate number of connected positroids on [n] to the total number of positroids through an inversion (see A134264)

Wardians, $W{n,k}$ (A134991, refined-A134685):

f-vectors of the Whitehouse simplicial complex associated with the tropical Grassmannians G(2,k) and phylogenetic trees (Bergman matroids?), related to enumeration of partitions of 2n objects.

Questions

A) Are there other perspectives on this tapestry involving these classic number clans, i.e., other ways in which these clans show up in the tapestry?

B) Can someone give a more cogent overview of these two threads and their relation to the classic number arrays?

References

(More details for the interested.)

Thread $T_V$:

1) Alcoved polytopes I, Lam and Postnikov, pages 1, 2, 18, and 21,

keywords: volumes, hypersimplices, Eulerian, grassmannian manifold, torus orbit

2) Matroid polytopes and volumes, Ardila, Benedetti, and Doker, page 6,

keywords: generalized permutohedra, grassmannian, torus orbit, volumes

3) Loops, Legs and Twistors, Spradlin,

keywords: contour integral, amplitudes, polynomials equations, Eulerian numbers

4) A note on polytopes for scattering amplitudes, Arkani-Hamed, Bourjaily, Cachazo, Hodges, and Trnka, pages 5-10,

keywords: twistor theory, volumes, areas, contour integral, differential forms

5) Scattering in three dimensions from rational maps, Cachazo, He, and Yuan, pages 4 and 11,

keywords: Eulerian numbers, scattering equations

6) Scattering Equations, Yuan

keywords: vanishing, quadratic differential, polynomoal maps, Eulerian numbers

7) Gravity in Twistor Space and its Grassmannian Formulation, Cachazo, Mason, and Skinner, pages 18 and 19,

keywords: Eulerian number, marked points

Thread $T_C$;

8) Matching polytopes, toric geometry, and the non-negative part of the Grassmannian, Postnikov, Speyer, and Williams, pages 1-2 and 12-13,

keywords: Grassmannians, matroid polytope, toric variety, cluster algebra

9) Cluster Polylogarithms for Scattering Amplitudes, Golden, Paulos, Spradlin, and Volovich, pages 8-13,

keywords: Stasheff associahedra, cluster functions

10) Studying Quantum Field Theory, Todorov, pages 17-20,

keywords: Catalan numbers, iterated integrals, simplices, polylogarithms

Additional notes on number clans, combinatorics, polytopes, and algebraic geometry:

11) On the Hilbert polynomials and Hilbert series of homogeneous projective varieties, Gross and Wallach, pages 13 and 14,

keywords: Grassmannian, Catalan numbers, Narayana numbers

12) Eulerian polynomials, Hirzebruch

13) New moduli spaces of pointed curves and pencils of flat connections, Losev and Manin, page 8, (Eulerians misnamed as the the Euler numbers)

14) For the Eulerian polynomial, see my recent entry in the formula section of A008292 and the associated links to Lenart and Zanoulline, and Buchstaber and Bunkova.

15) For the Narayana polynomials, see my recent example in A134264 and the associated reference to Ardila, Rincon, and Williams.

16) For enumeration of positroid cells of G+ and generating series interpolating between the Eulerians and Narayanaians, see A046802 and links therein.

17) Reflection group counting and q-counting, Reiner,

keywords: Catalan and Narayana numbers, parking functions, Weyl groups, q-extensions

Examining these brings to light another colorful thread in the tapestry related to moduli spaces, configuration spaces of particles, marked surfaces, and the polytopes associated to Lagrange inversion in different “coordinates”–o.g.f.s, e.g.f.s, etc. (The Lagrange inversion associated with the refined Eulerian partition polynomials seems to be “coordinate-free” in terms of the input in some sense.) Rather intriguing to me.