  We investigate the time evolution of the entanglement of assistance when one
subsystem undergoes the action of local noisy channels. A general factorization
law is presented for the evolution equation of entanglement of assistance. Our
results demonstrate that the dynamics of the entanglement of assistance is
determined by the action of a noisy channel on the pure maximally entangled
state, in which the entanglement reduction turns out to be universal for all
quantum states entering the channel. This single quantity will make it easy to
characterize the entanglement dynamics of entanglement of assistance under
unknown channels in the experimental process of producing entangled states by
assisted entanglement.
