  Based on the invariance principle of differential equations, a simple
adaptive-feedback scheme is proposed to strictly synchronize almost all chaotic
systems. Unlike the usual linear feedback, the variable feedback strength is
automatically adapted to completely synchronize two almost arbitrary identical
chaotic systems, so this scheme is analytical, and simple to implement in
practice. Moreover it is quite robust against the effect of noise. The famous
Lorenz and R$\ddot{\textrm{o}}$ssler hyperchaos systems are used as
illustrative examples.
