We investigate sets of the common zeros of non-constant semi-invariants for regular modules over canonical algebras. In particular, we show that if the considered algebra is tame then for big enough vectors these sets are complete intersections.Throughout the paper k denotes a fixed algebraically closed field of characteristic 0. By N and Z we denote the sets of non-negative integers and integers, respectively. Additionally, if i, j ∈ Z, then [i, j] = {l ∈ Z | i ≤ l ≤ j}.
Introduction and the main resultWith a finite dimensional algebra Λ and a dimension vector d we may associate the variety of Λ-modules of dimension vector d (see 2.1). An interesting problem investigated in the representation theory of finite dimensional algebras is the study of geometrical properties of these varieties (see, for example, [8,10,12,14,17,23,27,28,34,35,48]). In addition to this topic rings of semi-invariants (see 2.2) are also studied (see, for example, [20,24,30,32,40,46]). Recently, investigations of sets of the common zeros of non-constant semi-invariants were initiated by Chang and Weyman [15] and then continued by Riedtmann and Zwara [36][37][38][39]. Their investigations concerned situations of quivers without relations and were based on known results about semi-invariants in these cases (among others Sato-Kimura theorem [42]). An inspiration for their research was an observation that if, for a given dimension vector, the set of the common zeros of non-constant semi-invariants has a "good" codimension then the coordinate ring of the module variety is free as a module over the ring of semi-invariants.An important class of algebras are the canonical algebras introduced by Ringel [41, 3.7] (see 1.4). These algebras play an important role in representation theory (see, for example, [22,25,33,44]). Module varieties over canonical algebras were also studied [5,6]. One may distinguish a special class of modules over canonical algebras, called regular (see 1.6). The rings of semi-invariants for dimension vectors of regular modules over canonical algebra were