Trees of Fractions
摘要
This is an exposition on the submonoids of the monoid \( \mathsf {GL}_2 (\mathbb N) \) and the related trees of Calkin–Wilf, Stern–Brocot, Bird, Drib, Yu–Ting–Andreev, Hanna–Czyz–Self, Kepler and Lebesgue.