Welcome to my homepage! My name is Li Gan. I am a PhD graduate in physics at LPTMS, Université Paris-Saclay, under the supervision of Professor Stéphane Ouvry. My thesis mainly focuses on the enumeration of closed **lattice random walks** according to their **algebraic area** (a.k.a. signed area), with connections to **quantum exclusion statistics**, as well as the **combinatorics of generalized Dyck and Motzkin paths**.

** News**: I am delighted to give a seminar talk at LIPN, Université Sorbonne Paris Nord, in October 2023. Here are my slides.

*The most difficult thing is not facing setbacks and frustrations, but facing them without losing enthusiasm for life.* Ko Wen-je, former mayor of Taipei City

- Statistical mechanics
- Lattice random walks
- Enumerative combinatorics
- Exactly solvable models
- Combinatorial physics

PhD in Physics, 2024

Université Paris-Saclay

Diplôme d'Ingénieur, 2018

École Centrale Paris

BSc in Mathematics, 2015

École Centrale de Pékin

We obtain an explicit formula to enumerate closed random walks on a cubic lattice with a specified length and algebraic area. The algebraic area of a closed cubic lattice walk is defined as the sum of the algebraic areas obtained from the walk’s projection onto the three Cartesian planes. This enumeration formula can be mapped onto the cluster coefficients of three types of particles that obey quantum exclusion statistics with statistical parameters $g=1$, $g=1$, and $g=2$, respectively, subject to the constraint that the numbers of $g=1$ (fermions) exclusion particles of two types are equal.

We relate the combinatorics of periodic generalized Dyck and Motzkin paths to the cluster coefficients of particles obeying generalized exclusion statistics, and obtain explicit expressions for the counting of paths with a fixed number of steps of each kind at each vertical coordinate. A class of generalized compositions of the integer path length emerges in the analysis.

We study the enumeration of closed walks of given length and algebraic area on the honeycomb lattice. Using an irreducible operator realization of honeycomb lattice moves, we map the problem to a Hofstadter-like Hamiltonian and show that the generating function of closed walks maps to the grand partition function of a system of particles with exclusion statistics of order $g=2$ and an appropriate spectrum, along the lines of a connection previously established by two of the authors. Reinterpreting the results in terms of the standard Hofstadter spectrum calls for a mixture of $g=1$ (fermion) and $g=2$ exclusion particles whose properties merit further studies. In this context we also obtain some unexpected Fibonacci sequences within the weights of the combinatorial factors appearing in the counting of walks.