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We present fast algorithms for computing rational first integrals with bounded degree of a planar polynomial vector field. Our approach builds upon a method proposed by Ferragut and Giacomini, whose main ingredients are the calculation of a power series solution of a first order differential equation and the reconstruction of a bivariate polynomial annihilating this power series. We provide explicit bounds on the number of terms needed in the power series. This enables us to transform their method into a certified algorithm computing rational first integrals via systems of linear equations. We then significantly improve upon this first algorithm by building a probabilistic algorithm with arithmetic complexity $~O(N^{2 omega})$ and a deterministic algorithm solving the problem in at most $~O(d^2N^{2 omega+1})$ arithmetic operations, where~$N$ denotes the given bound for the degree of the rational first integral, and where $d leq N$ is the degree of the vector field, and $omega$ the exponent of linear algebra. We also provide a fast heuristic variant which computes a rational first integral, or fails, in $~O(N^{omega+2})$ arithmetic operations. By comparison, the best previous algorithm uses at least $d^{omega+1}, N^{4omega +4}$ arithmetic operations. We then show how to apply a similar method to the computation of Darboux polynomials. The algorithms are implemented in a Maple package RationalFirstIntegrals which is available to interested readers with examples showing its efficiency.

Efficient algorithms for computing rational first integrals and Darboux polynomials of planar polynomial vector fields