1. Ambrosio, L., Kirchheim, B.: Rectifiable sets in metric and banach spaces. Mathematische Annalen 318(3), 527–555 (2000)
2. Arcozzi, N., Morbidelli, D.: Stability of isometric maps in the Heisenberg group. Comment. Math. Helv. 83(1), 101–141 (2008)
3. Berestovskiĭ, V.N.: Geodesics of nonholonomic left-invariant inner metrics on the Heisenberg group and isoperimetrics of the Minkowski plane. Sibirsk. Mat. Zh. 35(1), 3–11 (1994)
4. Bridson, M.R., Haefliger, A.: Metric spaces of non-positive curvature. Grundlehren der Mathematischen Wissenschaften [Fundamental Principles of Mathematical Sciences], vol. 319. Springer, Berlin (1999)
5. Burago, D., Burago, Y., Ivanov, S.: A Course in Metric Geometry. Vol. 33 of Graduate Studies in Mathematics. American Mathematical Society, Providence (2001)