Chen, J., P. Hariri, R. Klén, and M. Vuorinen. “Lipschitz Conditions, Triangular Ratio Metric, and Quasiconformal Maps”. Annales Fennici Mathematici, vol. 40, no. 2, Aug. 2015, pp. 683-09, doi:10.5186/aasfm.2015.4039.