CMSA Colloquium: Snake graphs and q-deformed real numbers
CMSA COLLOQUIUM
When: November 16, 2026
4:30 pm - 5:30 pm
Where: CMSA, 20 Garden St, G10
Address:
20 Garden Street, Cambridge, MA 02138, United States
Speaker: James Propp (UMass Lowell)
I’ll present a statistical-mechanical construction of q-deformed real numbers [[x]]_q, inspired by the q-deformed rational numbers [x]_q of Morier-Genoud and Ovsienko. The new construction admits two equivalent formulations: one in terms of random filters in snake posets and the other in terms of random dimer covers of snake graphs.
I’ll begin with a combinatorial approach to Stern’s diatomic series and the Calkin–Wilf bijection via finite snake graphs, and then construct q-deformed rational numbers by introducing an activity parameter q into the model. I extend from q-deformed rationals to q-deformed reals by passing from finite to infinite snake graphs; the main technical step is to show that the associated limiting probability measures are well defined.
The resulting q-deformation [[x]]_q satisfies [[x]]_1 = x for all x>0. There’s evidence that, wherever both [x]_q and [[x]]_q are defined, [[x]]_q = q [x]_q. For more details, see arXiv:2607.14332.