Clustertilting theory.
EPSRC Grant EP/G007497/1
This project ran from September 2008 to August 2013 (extended to March 2014) and was funded by the UK Engineering and Physical Sciences Research Council, 841,641 pounds.
Leadership Fellow: Robert Marsh.
Research Fellows: Yann Palu, Joseph Grant.
Summary of Project (nonspecialist)
Manifolds
A manifold is a mathematical space which is locally like the usual coordinate space of some fixed dimension, known as Euclidean space. The earth itself is an example of a twodimensional manifold: maps exploit the fact that, if you don't go too far from a given point, the earth behaves pretty much like a plane. The earth could be covered entirely by small maps making use of this local planar structure, although an accurate map of the entire world must be a globe. A onedimensional manifold, such as a circle, looks locally like a line, and there are manifolds of higher dimension as well. Manifolds play an important role in mathematics because there are so many examples and because they can be studied using the properties of Euclidean space.
Symmetries
The symmetries of an object are the transformations which leave it looking exactly as it was before, such as a rotation of a square through a quarter of a revolution. Two symmetries can always be composed to give a third. This, together with other properties, gives the set of symmetries of any object the structure of a group. While there are only 8 symmetries of a square, the circle has infinitely many symmetries, forming a onedimensional manifold. The manifold and group structures are compatible, and such an object is known as a Lie group. Lie groups often arise as certain symmetry groups known as gauge groups in physics as well.
Lie groups
In the 1870s, Sophus Lie studied Lie groups by linearizing them, to obtain simpler spaces known as Lie algebras. Lie algebras can be studied using more elementary mathematical notions from linear algebra, which is the study of vector spaces (such as Euclidean spaces) and their linear transformations. This and other developments led to the modern subject of Lie theory in which Lie algebras are studied via their representation theory: each element of the Lie algebra is represented by a transformation of a vector space. Such transformations can be represented more concretely via matrices, which are rectangular arrays of numbers, provided a basis is chosen (essentially a choice of coordinates).
Quantum groups
In 1985, deformed versions of the classical Lie algebras, or quantum groups, were introduced by Drinfeld and Jimbo, and this led to a revolution in the way that Lie algebras were studied. In particular, Kashiwara and Lusztig introduced the canonical basis in 1990, with beautiful properties. It simultaneously gives rise to bases for all representations for the Lie algebra (of a certain kind). The canonical basis has applications to other fields as well, such as the representation theory of affine Hecke algebras.
The project
Attempts to describe the canonical basis explicitly have led to a lot of interesting mathematics, including the cluster algebras of Fomin and Zelevinsky, introduced to model its multiplicative properties. Cluster categories and clustertilting theory were introduced in order to understand cluster algebras. These objects were defined using representations of quivers: a quiver is a graph with oriented edges and a representation is a vector space for each vertex and a transformation for each edge. As well as giving insight into the canonical basis, clustertilting theory has rapidly become a powerful tool for the study of representations of quivers.
This model of the canonical basis is still incomplete and work on the proposed project will help towards a better picture via quantum cluster algebras. The project will also work on developing clustertilting theory from the perspective of representations of quivers by describing its combinatorial properties and generalising it to a wider context in several directions. The project will also develop stronger connections to the diagram algebras arising in statistical mechanics.
Supported outputs
 R. J. Marsh and K. Rietsch. The Bmodel connection and mirror symmetry for Grassmannians. Advances in Mathematics 366, 107027 (2020). (Preprint version, January 2020)
 R. J. Marsh, Lecture Notes on Cluster Algebras. Zurich lectures in Advanced Mathematics. European Mathematical Society, 2014.
 A. B. Buan and R. J. Marsh, τexceptional sequences, Preprint, February 2018.
 R. J. Marsh and I. Reiten. Rigid and Schurian modules over clustertilted algebras of tame type. Mathematische Zeitschrift, 284(3), 643682, 2016. (Click here for Preprint version).
 R. J. Marsh and J. Grant. Braid groups and quiver mutation. Pacific Journal of Mathematics 2901 (2017), 77116 (to appear). Preprint version (3/2017).
 K. Baur, A. King, and R. J. Marsh. Dimer models and cluster categories of Grassmannians. Proc. London Math. Soc. 113(2), 213260, 2016. (Preprint version, June 2016).
 R. J. Marsh and Y. Palu. Nearly Morita equivalences and rigid objects. Nagoya Mathematical Journal, 225, 6499, 2017. Preprint version (2/2016).
 R. J. Marsh and J. S. Scott. Twists of Plücker coordinates as dimer partition functions. Comm. Math. Phys. 341 Issue 3 (2016), 821884. Preprint version
 R. J. Marsh and S. Schroll. The geometry of Brauer graph algebras and cluster mutations. J. Algebra 419, (2014), 141166. Preprint version.
 R. J. Marsh and K. Rietsch. The Bmodel connection and mirror symmetry for Grassmannians. Preprint, July 2013. Revised December 2015.

K. Baur,
A. B. Buan and
R. J. Marsh.
Torsion pairs and rigid objects in tubes.
Algebras and Representation Theory (2014) 17(2): 565591. Preprint. The final publication is available at link.springer.com.  M. Barot and R. J. Marsh. Reflection group presentations arising from cluster algebras. Trans. Amer. Math. Soc. 367 (2015), 19451967. Preprint version.
 A. B. Buan and R. J. Marsh. From triangulated categories to module categories via localization II: Calculus of fractions. J. London Math. Soc. (2012) 86 (1): 152170. OPEN ACCESS.
 R. J. Marsh and Y. Palu. Coloured quivers for rigid objects and partial triangulations: The unpunctured case. Proceedings of the London Mathematical Society (2014) 108 (2), 411440. Preprint version. Freeaccess link.
 K. Baur and R. J. Marsh. A geometric model of tube categories. J. Algebra 362 (2012), 178191. Preprint.
 R. J. Marsh and S. Schroll. A circular order on edgecoloured trees and RNA mdiagrams. Adv Appl. Math. 54 (2014), 1126. Adv. Appl. Math 54 (2014), 1126. Preprint version.

A. B. Buan and
R. J. Marsh.
From triangulated categories to module categories via localisation.
Trans. Amer. Math. Soc. 365, no. 6 (2013), 28452861. Preprint.  K. Baur and R. J. Marsh. Categorification of a frieze pattern determinant. Journal of Combinatorial Theory, Series A 119 (2012), 11101122. Preprint.
 P. Jorgensen and Y. Palu. A CalderoChapoton map for infinite clusters. Trans. Amer. Math. Soc. 365 (2013), no. 3, 11251147.

R. J. Marsh and
P. P. Martin.
Tiling bijections between paths and Brauer diagrams.
Journal of Algebraic Combinatorics, 33, no. 3 (2011), 427453. Preprint. 
A. P. Fordy and
R. J. Marsh.
Cluster mutationperiodic quivers and associated Laurent sequences.
Journal of Algebraic Combinatorics, 34, no. 1 (2011), 1966. Preprint. 
A. B. Buan,
R. J. Marsh and
D. F. Vatne.
Cluster structures from 2CalabiYau categories with loops.
Mathematische Zeitschrift 265 no. 4 (2010), pages 951970. Preprint.