@TechReport{BKMS08,
    author        = {Hans-Joachim B{\"o}ckenhauer and Ralf Klasing and Tobias
                    M{\"o}mke and Monika Steinov{\'a}},
    title         = {Improved Approximations for {TSP} with Simple Precedence
                    Constraints},
    institution   = {ETH Z{\"u}rich},
    number        = {650},
    year          = {2009},
}