1. Ball, D. L. , & Bass, H. (2009). With an eye on the mathematical horizon: Knowing mathematics for teaching to learners’ mathematical futures. In M. Neubrand , (Ed.), Beiträge zum Mathematikunterricht 2009. Münster: WTM-Verlag. Retrieved from www.mathematik.uni-dortmund.de/ieem/BzMU/BzMU2009/Beitraege/Hauptvortraege/BALL_Deborah_BASS_Hyman_2009_Horizon.pdf