Résumé
We prove that given an undirected graph G on n vertices and an integer k, one can compute in polynomial time in n a graph G' with at most 5k2 + k vertices and an integer k' such that G has a feedback vertex set of size at most k iff G' has a feedback vertex set of size at most k'. This result improves a previous O(k11) kernel of Burrage et al. [6], and a more recent cubic kernel of Bodlaender [3]. This problem was communicated by Fellows in [5].