In an explicit family of partially hyperbolic diffeomorphisms of the torus T 3, Shub andWilkinson recently succeeded in perturbing the Lyapunov exponents of the center direction. We present here a local version of their argument, allowing one to perturb the center Lyapunov exponents of any partially hyperbolic system, in any dimension and with arbitrary dimension of the center bundle.
Ergodic theory and dynamical systems. Cambridge. Vol. 23, no. 6 (2003), p. 1655-1670.