Hyperbolic Lambert Quadrilaterals and Quasiconformal Mappings
  We prove sharp bounds for the product and the sum of two hyperbolic distances between the opposite sides of hyperbolic Lambert quadrilaterals in the unit disk. Furthermore, we study the images of Lambert quadrilaterals under quasiconformal mappings from the unit disk onto itself and obtain sharp results in this case, too. 
Matti Vuorinen and Gendi Wang
math.MG