The following publications are possibly variants of this publication:
- Guarding a terrain by two watchtowersPankaj K. Agarwal, Sergey Bereg, Ovidiu Daescu, Haim Kaplan, Simeon C. Ntafos, Binhai Zhu. compgeom 2005: 346-355 [doi]
- On the Planar Two-Watchtower ProblemSergei Bespamyatnikh, Zhixiang Chen, Kanliang Wang, Binhai Zhu. cocoon 2001: 121-130 [doi]
- The Shortest Watchtower and Related Problems for Polyhedral TerrainsMicha Sharir. ipl, 29(5):265-270, 1988.