William C. K. Yen C. Y. Tang An Optimal Algorithm for Solving the Searchlight Guarding Problem on Weighted Two-Terminal Series-Parallel Graphs. 143-172 1999 36 Acta Inf. 2 http://link.springer.de/link/service/journals/00236/bibs/9036002/90360143.htm db/journals/acta/acta36.html#YenT99