The generic Groebner walk

The Groebner walk is an algorithm for conversion between Groebner bases for different term orders. It is based on the polyhedral geometry of the Groebner fan and involves tracking a line between cones representing the initial and target term order. An important parameter is explicit numerical perturbation of this line. This usually involves both time and space demanding arithmetic of integers much larger than the input numbers. In this paper we show how the explicit line may be replaced by a formal line using Robbiano's characterization of group orders on Q^n. This gives rise to the generic Groebner walk involving only Groebner basis conversion over facets and computations with marked polynomials. The proposed technique is closely related to the lexicographic (symbolic) perturbation method used in optimization and computational geometry. We report on computations with toric ideals, where a version of our algorithm in certain cases computes test sets for hard integer knapsack problems significantly faster than the Buchberger algorithm.