Résumé
Efficient block projections of non-singular matrices have recently been used by the authors in [10] to obtain an efficient algorithm to find rational solutions for sparse systems of linear equations. In particular a bound ofO~(n2.5) machine operations is presented for this computation assuming that the input matrix can be multiplied by a vector with constant-sized entries using O~(n) machine operations. Somewhat more general bounds for black-box matrix computations are also derived. Unfortunately, the correctness of this algorithm depends on the existence of efficient block projections of non-singular matrices, and this was only conjectured.
In this paper we establish the correctness of the algorithm from [10] by proving the existence of efficient block projections for arbitrary non-singular matrices over sufficiently large fields. We further demonstrate the usefulness of these projections by incorporating them into existing black-box matrix algorithms to derive improved bounds for the cost of several matrix problems. We consider, in particular, matrices that can be multiplied by a vector using O~(n) field operations: We show how to compute the inverse of any such non-singular matrix over any field using an expected number of O~(n2.27) operations in that field. A basis for the null space of such a matrix, and a certification of its rank, are obtained at the same cost. An application of this technique to Kaltofen and Villard's Baby-Steps/Giant-Steps algorithms for the determinant and Smith Form of an integer matrix is also sketched, yielding algorithms requiring O~(n2.66) machine operations. More general bounds involving the number of black-box matrix operations to be used are also obtained.
The derived algorithms are all probabilistic of the Las Vegas type. They are assumed to be able to generate random elements - bits or field elements - at unit cost, and always output the correct answer in the expected time given.