Alt, H., Fleischer, R., Kaufmann, M., Mehlhorn, K., Näher, S., Schirra, S., & Uhrig, C. (1990). Approximate Motion Planning and the Complexity of the Boundary of the Union of Simple Geometric Figures. In Computational geometry (SCG-90): 6th annual symposium (pp. 281-289). New York, USA: ACM.