Equilateral triangle skew condition for quasiconformality Colleen AckermannPeter HaissinskyAimo Hinkkanen Indiana University Mathematics Journal 2019 text pdf 10.1512/iumj.2019.68.7762 10.1512/iumj.2019.68.7762 en Indiana Univ. Math. J. 68 (2019) 1591 - 1608 state-of-the-art mathematics http://iumj.org/access/