Gentle algebras arising from surfaces with orbifold points, Part II: Locally free Caldero–Chapoton functions

We prove that in the skew-symmetrizable cluster algebras associated by Felikson–Shapiro–Tumarkin to unpunctured surfaces with orbifold points of order 2 and a specific choice of weights, the Laurent expansion of any cluster variable with respect to any cluster coincides with the locally free Caldero–Chapoton function of a τ -rigid representation of a gentle algebra. These cluster algebras are typically non-acyclic and of infinite type, whereas for polygons with one orbifold point one recovers cluster algebras of finite type C ; so, our result is an ample extension of a seminal result established by Geiss–Leclerc–Schröer for skew-symmetrizable cluster algebras of finite type and acyclic initial seeds. As the main means to achieve the result, we provide a generalization of Derksen–Weyman–Zelevinsky's mutation theory of loop-free quivers with potential to the quivers-with-loops with potential we associate to the triangulations of unpunctured surfaces with orbifold points, and study the relation with τ -tilting theory. As a result of independent interest, we compute the aforementioned τ -rigid representations explicitly. To this end, we show that the indecomposable τ -rigid string modules arising from arcs on the surface, and the quasi-simple band modules arising from simple closed curves, are well-behaved under the mutations of representations we define in the paper, thus extending results of the first author's Ph.D. thesis.