EXPONENTIAL STABILITY FOR SOLUTIONS OF CONTINUOUS AND DISCRETE ABSTRACT CAUCHY PROBLEMS IN BANACH SPACES

Let T be a strongly continuous semigroup acting on a complex Banach space X and let A be its infinitesimal generator. It is well-known [29, 33] that the uniform spectral bound s(0)(A) of the semigroup T is negative provided that all solutions to the Cauchy problems(u) over dot(t) = Au(t) + e(i mu t)x, t = 0, u(0) = 0,are bounded (uniformly with respect to the parameter mu is an element of R). In particular, if X is a Hilbert space, then this yields all trajectories of the semigroup T are exponentially stable, but if X is an arbitrary Banach space this result is no longer valid. Let chi denote the space of all continuous and 1-periodic functions f : B - X whose sequence of Fourier-Bohr coefficients (c(m)(f ))(m)is an element of Z belongs to l(1)(Z, X). Endowed with the norm parallel to f parallel to(1) = parallel to(c(m)(f))(m)is an element of Z parallel to(1) it becomes a non-reflexive Banach space [15]. A subspace A(T) of X (related to the pair (T, chi)) is introduced in the third section of this paper. We prove that the semigroup T is uniformly exponentially stable provided that in addition to the above-mentioned boundedness condition, A(T) = X. An example of a strongly continuous semigroup (which is not uniformly continuous) and fulfills the second assumption above is also provided. Moreover an extension of the above result from semigroups to 1-periodic and strongly continuous evolution families acting in a Banach space is given. We also prove that the evolution semigroup T associated with T on chi does not verify the spectral determined growth condition. More precisely, an example of such a semigroup with uniform spectral bound negative and uniformly growth bound non-negative is given. Finally we prove that the assumption A(T) = X is not needed in the discrete case.