Dominant root sequences of arbitrary degree

Abstract

We show that for root-subgroup multiplication maps, dominance does not imply birationality. This gives a negative answer to a question of Fang, Fourier and Littelmann. More precisely, for every integer $r\geq 2$, we construct a sequence of positive roots in type $A_{3r-1}$; the associated multiplication map is dominant of generic degree $r$, hence not birational. In our construction, a key observation is that Lusztig's flag incidence projection for a triangle quiver is, up to birational equivalence and identity factors, simultaneously a root-subgroup multiplication map and a multiplicative partial Grothendieck--Springer map. Moreover, we extend these examples to types $B$, $C$ and $D$ in every generic degree $r\geq 2$, and to types $E_6$, $E_7$ and $E_8$ in generic degree $2$.