In 2002,Faugère presented the famous F5 algorithm for computing Grbner basis where two criteria,syzygy criterion and rewritten criterion,were proposed to avoid redundant computations.He proved the correctness of the syzygy criterion,but the proof for the correctness of the rewritten criterion was left.Since then,F5 has been studied extensively.Some proofs for the correctness of F5 were proposed,but these proofs are valid only under some extra assumptions.In this paper,we give a proof for the correctness of F5B,an equivalent version of F5 in Buchberger's style.The proof is valid for both homogeneous and non-homogeneous polynomial systems.Since this proof does not depend on the computing order of the S-pairs,any strategy of selecting S-pairs could be used in F5B or F5.Furthermore,we propose a natural and non-incremental variant of F5 where two revised criteria can be used to remove almost all redundant S-pairs.
An efficient algorithm is proposed for factoring polynomials over an algebraic extension field defined by a polynomial ring modulo a maximal ideal. If the maximal ideal is given by its Grbner basis, no extra Grbner basis computation is needed for factoring a polynomial over this extension field. Nothing more than linear algebraic technique is used to get a characteristic polynomial of a generic linear map. Then this polynomial is factorized over the ground field. From its factors, the factorization of the polynomial over the extension field is obtained. The algorithm has been implemented in Magma and computer experiments indicate that it is very efficient, particularly for complicated examples.
