  In this paper, we address the problem of safety verification of nonlinear
hybrid systems. A hybrid symbolic-numeric method is presented to compute exact
inequality invariants of hybrid systems efficiently. Some numerical invariants
of a hybrid system can be obtained by solving a bilinear SOS programming via
PENBMI solver or iterative method, then the modified Newton refinement and
rational vector recovery techniques are applied to obtain exact polynomial
invariants with rational coefficients, which {\it exactly} satisfy the
conditions of invariants. Experiments on some benchmarks are given to
illustrate the efficiency of our algorithm.
