Stéphane Bessy Fedor V. Fomin Serge Gaspers Christophe Paul Anthony Perez Saket Saurabh Stéphan Thomassé Kernels for Feedback Arc Set In Tournaments http://arxiv.org/abs/0907.2165 2009 CoRR abs/0907.2165 db/journals/corr/corr0907.html#abs-0907-2165 informal publication