Table of Contents

1 Matroids and Flag Matroids.- 1.1 Matroids.- 1.2 Representable matroids.- 1.3 Maximality Property.- 1.4 Increasing Exchange Property.- 1.5 Sufficient systems of exchanges.- 1.6 Matroids as maps.- 1.7 Flag matroids.- 1.8 Flag matroids as maps.- 1.9 Exchange properties for flag matroids.- 1.10 Root system.- 1.11 Polytopes associated with flag matroids.- 1.12 Properties of matroid polytopes.- 1.13 Minkowski sums.- 1.14 Exercises for Chapter 1.- 2 Matroids and Semimodular Lattices.- 2.1 Lattices as generalizations of projective geometry.- 2.2 Semimodular lattices.- 2.3 Jordan—Hölder permutation.- 2.4 Geometric lattices.- 2.5 Representations of matroids.- 2.6 Representation of flag matroids.- 2.7 Every flag matroid is representable.- 2.8 Exercises for Chapter 2.- 3 Symplectic Matroids.- 3.1 Definition of symplectic matroids.- 3.2 Root systems of type Cn.- 3.3 Polytopes associated with symplectic matroids.- 3.4 Representable symplectic matroids.- 3.5 Homogeneous symplectic matroids.- 3.6 Symplectic flag matroids.- 3.7 Greedy Algorithm.- 3.8 Independent sets.- 3.9 Symplectic matroid constructions.- 3.10 Orthogonal matroids.- 3.11 Open problems.- 3.12 Exercises for Chapter 3.- 4 Lagrangian Matroids.- 4.1 Lagrangian matroids.- 4.2 Circuits and strong exchange.- 4.3 Maps on orientable surfaces.- 4.4 Exercises for Chapter 4.- 5 Reflection Groups and Coxeter Groups.- 5.1 Hyperplane arrangements.- 5.2 Polyhedra and polytopes.- 5.3 Mirrors and reflections.- 5.4 Root systems.- 5.5 Isotropy groups.- 5.6 Parabolic subgroups.- 5.7 Coxeter complex.- 5.8 Labeling of the Coxeter complex.- 5.9 Galleries.- 5.10 Generators and relations.- 5.11 Convexity.- 5.12 Residues.- 5.13 Foldings.- 5.14 Bruhat order.- 5.15 Splitting the Bruhat order.- 5.16 Generalized permutahedra.- 5.17 Symmetricgroup as a Coxeter group.- 5.18 Exercises for Chapter 5.- 6 Coxeter Matroids.- 6.1 Coxeter matroids.- 6.2 Root systems.- 6.3 The Gelfand—Serganova Theorem.- 6.4 Coxeter matroids and polytopes.- 6.5 Examples.- 6.6 W-matroids.- 6.7 Characterization of matroid maps.- 6.8 Adjacency in matroid polytopes.- 6.9 Combinatorial adjacency.- 6.10 The matroid polytope.- 6.11 Exchange groups of Coxeter matroids.- 6.12 Flag matroids and concordance.- 6.13 Combinatorial flag variety.- 6.14 Shellable simplicial complexes.- 6.15 Shellability of the combinatorial flag variety.- 6.16 Open problems.- 6.17 Exercises for Chapter 6.- 7 Buildings.- 7.1 Gaussian decomposition.- 7.2 BN-pairs.- 7.3 Deletion Property.- 7.4 Deletion property and Coxeter groups.- 7.5 Reflection representation of W.- 7.6 Classification of finite Coxeter groups.- 7.7 Chamber systems.- 7.8 W-metric.- 7.9 Buildings.- 7.10 Representing Coxeter matroids in buildings.- 7.11 Vector-space representations and building representations.- 7.12 Residues in buildings.- 7.13 Buildings of type An-1 = Symn.- 7.14 Combinatorial flag varieties, revisited.- 7.15 Open Problems.- 7.16 Exercises for Chapter 7.- References.