Reinhard Bauer Gianlorenzo D'Angelo Daniel Delling Dorothea Wagner The Shortcut Problem - Complexity and Approximation. 105-116 2009 SOFSEM https://doi.org/10.1007/978-3-540-95891-8_13 conf/sofsem/2009 db/conf/sofsem/sofsem2009.html#BauerDDW09