Dr. Matthias Hamann

My research interests lie in algebraic and structural graph and digraph theory and geometric group theory. A (not complete) list of more specific topics is the following:

• Cayley graphs
• tree decompositions
• hyperbolic spaces
• graphs of groups
• coarse geometry of transitive graphs
• homogeneous structures

• with K. Heuer
  Infinite grids in digraphs
  European Journal of Combinatorics 135 (2026), Paper No. 104370
  
  
• with S. Albrechtsen
  Asymptotic half-grid and full-grid minors
  Journal of Combinatorial Theory (Series B) 176 (2026), 440-485
  
  
• with T. Abrishami, L. Esperet, U. Giocanti, P. Knappe, R.G. Möller
  Periodic colorings and orientations in infinite graphs
  Combinatorial Theory 5 (2025), #5
  
  
• Tree amalgamations and quasi-isometries
  Journal of Combinatorial Theory (Series B) 173 (2025), 83-101
  
  
• Quasi-transitive K_∞-minor free graphs
  European Journal of Combinatorics 124 (2025), Paper No. 104056
  
  
• with A. Georgakopoulos
  A full Halin grid theorem
  Discrete & Computational Geometry (2024)
  
  
• Minor exclusion in quasi-transitive graphs
  Discrete Mathematics 347 (2024), 113738
  
  
• with A. Georgakopoulos
  The planar Cayley graphs are effectively enumerable II
  European Journal of Combinatorics 110 (2023), Paper No. 103668
  
  
• with F. Lehner, B. Miraftab & T. Rühmann
  A Stallings type theorem for quasi-transitive graphs
  Journal of Combinatorial Theory (Series B) 157 (2022), 40-69
  
  
• with J. Carmesin & B. Miraftab
  Canonical trees of tree-decompositions
  Journal of Combinatorial Theory (Series B) 152 (2022), 1-26
  
  
• with A. Georgakopoulos
  The planar Cayley graphs are effectively enumerable I: consistently planar graphs
  Combinatorica 39 (2019), 993-1019
  
  
• Self-embeddings of trees
  Discrete Mathematics 342 (2019), 111586
  
  
• Planar transitive graphs
  Electronic Journal of Combinatorics 25 (2018), Paper 4.8
  
  
• Accessibility in transitive graphs
  Combinatorica 38 (2018), 847-859
  
  
• On the tree-likeness of hyperbolic spaces
  Math. Proc. Cambridge Philos. Soc. 164 (2018), 345-361
  
  
• The classification of finite and locally finite connected-homogeneous digraphs
  Combinatorica 37 (2017), 183-222
  
  
• with D. Weißauer
  Bounding connected tree-width
  SIAM Journal on Discrete Mathematics 30 (2016), 1391-1400
  
  
• Groups acting on metric spaces: fixed points and free subgroups
  Abhandlungen des Mathematischen Seminars der Universität Hamburg 87 (Special Issue: Rudolf Halin) (2017), 245-263
  
  
• with F. Lehner & J. Pott
  Extending cycles locally to Hamilton cycles
  Electronic Journal of Combinatorics 23 (2016), Paper 1.49
  
  
• with J. Carmesin, R. Diestel & F. Hundertmark
  Canonical tree-decompositions of finite graphs II. Essential parts
  Journal of Combinatorial Theory (Series B) 118 (2016), 268-283
  
  
• with J. Carmesin, R. Diestel & F. Hundertmark
  Canonical tree-decompositions of finite graphs I. Existence and algorithms
  Journal of Combinatorial Theory (Series B) 116 (2016), 1-24
  
  
• Spanning trees in hyperbolic graphs
  Combinatorica 36 (2016), 313-332
  
  
• Generating the cycle space of planar graphs
  Electronic Journal of Combinatorics 22 (2015), Paper 2.34
  
  
• with J. Carmesin, R. Diestel & F. Hundertmark
  k-Blocks: a connectivity invariant for graphs
  SIAM Journal on Discrete Mathematics 28 (2014), 1876-1891
  extended version
  
  
• Homogeneous 2-partite digraphs
  Discrete Mathematics 327 (2014), 36-39
  
  
• with F. Hundertmark
  The classification of connected-homogeneous digraphs with more than one end
  Transactions of the AMS 365 (2013), 531-553
  
  
• with J. Pott
  Transitivity conditions in infinite graphs
  Combinatorica 32 (2012), 649-688
  
  
• with A. Georgakopoulos
  On fixing boundary points of transitive hyperbolic graphs
  Archiv der Mathematik 99 (2012), 91-99
  
  
• End-transitive graphs
  Israel Journal of Mathematics 189 (2012), 437-459
  
  

Preprints

• Countable connected-homogeneous digraphs
  submitted
  
  
• Tree amalgamations and hyperbolic boundaries
  submitted
  
  
• Asymptotic dimension of multi-ended quasi-transitive graphs
  submitted
  
  
• with B. Miraftab
  Two characterisations of accessible quasi-transitive graphs
  submitted
  
  
• Quasi-isometry invariance of hyperbolicity in semimetric spaces, digraphs and semigroups
  submitted
  
  
• A boundary for hyperbolic digraphs and semigroups
  submitted
  
  
• Free submonoids of hyperbolic monoids
  submitted
  
  
• with K. Heuer
  An end degree for digraphs
  submitted
  
  
• with S. Albrechtsen
  A coarse Halin Grid Theorem with applications to quasi-transitive, locally finite graphs
  submitted
  
  
• with K. Heuer
  On ubiquity problems in infinite digraphs
  submitted
  
  

Theses

• Connected-homogeneous digraphs
  Habilitationsschrift, University of Hamburg, Department of Mathematics, September 2014
  
  
• Infinite graphs with a treelike structure
  PhD Thesis, University of Hamburg, Department of Mathematics, June 2011
  
  
• Über die lokale Struktur endlicher Gruppen lokaler Charakteristik p vom Rang 2
  Diplomarbeit, Christian-Albrechts-Universität zu Kiel, Mathematisches Seminar, October 2007
  
  

• Asymptotic grid minors in infinite graphs
  Kolkom 2025, Bielefeld (2025)
  
  
• Asymptotic half-grid and full-grid minors
  EUROComb 2025, Budapest (2025)
  
  
• Grid minors in transitive graphs
  AlgoLoG Seminar, DTU, Lyngby (2025)
  Research Seminar in Discrete Mathematics and Algebra, Freiberg (2025)
  
  
• Hyperbolic digraphs
  Kolkom 2024, Heidelberg (2024)
  
  
• Hyperbolic digraphs and monoids
  Colloquium of the Mathematics Department, Reykjavik (2024)
  AlgoLoG Seminar, DTU, Lyngby (2024)
  
  
• Splitting quasi-transitive infinite graphs
  Colloquium, University of Memphis (2022)
  
  
• Hyperbolic digraphs
  Forschungsseminar Diskrete Mathematik, Universität Hamburg (2021)
  
  
• Accessibility and canonical tree-decompositions
  3in1 Workshop on Graph Theory, Dosłońce (2017)
  
  
• Cuts, cycles and accessibility
  Alfréd Rényi Institute of Mathematics, Budapest (2016)
  
  
• Accessibility in transitive graphs
  Oberwolfach Workshop on Graph Theory (2016)
  
  
• Connected-homogeneous digraphs
  BIRS Workshop on homogeneous structures, Banff (2015)
  A video of this talk
  
  
• Cuts and cycles in transitive graphs
  Kolkom 2015, Ilmenau (2015)
  
  
• Accessible groups and graphs
  Colloquium of the Mathematics Department, Reykjavik (2015)
  
  
• Homogeneity in graphs and digraphs
  Pure Mathematics Colloquium, Southampton (2015)
  
  
• Accessibility in transitive graphs
  Workshop on Structure in Graphs and Matroids, Princeton (2014)
  
  
• Canonical tree structure of graphs
  Combinatorics Seminar, University of Warwick (2014)
  
  
• Hyperbolic graphs
  Oberwolfach Workshop on Graph Theory (2013)
  
  
• Homogeneity in infinite graphs
  2nd Workshop on Homogeneous Structures, Prague (2012)
  
  
• Connected-homogeneous digraphs
  LMS Northern Regional Meeting and Workshop on Homogeneous Structures, Leeds (2011)
  
  
• On the tree-likeness of hyperbolic graphs
  DIMAP Workshop on Combinatorics and Graph Theory, University of Warwick (2011)
  
  
• Hyperbolic graphs and trees
  Colloquium of the Mathematics Department, Reykjavik (2011)
  
  

Education

• 09/2014 Habilitation University of Hamburg
• 07/2011 Dr. rer. nat. University of Hamburg (Supervisor: Reinhard Diestel)
• 10/2007 German Diplom (Mathematik) Christian-Albrechts-University in Kiel

Professional experience

• since 10/2022 University of Hamburg
• 07/2019-09/2022 University of Warwick
• 10/2016-06/2019 Alfred Renyi Institute, Budapest
• 07/2011-09/2016 University of Hamburg

Scholarships and grants

• 10/2024-09/2026 Research Grant (DFG) "Splitting quasi-transitive graphs"
• 10/2017-09/2022 Heisenberg-Fellowship (DFG) "Symmetric and treelike graphs"
• 04/2008-06/2011 State graduate funding program scholarship

Research stays

DTU, Denmark
University of Iceland
University of Warwick
TU Graz