Let S be the shift operator on the Hardy space H2 and let Sβ be its
adjoint. A closed subspace \FF of H2 is said to be nearly Sβ-invariant
if every element f\in\FF with f(0)=0 satisfies S^*f\in\FF. In particular,
the kernels of Toeplitz operators are nearly Sβ-invariant subspaces. Hitt
gave the description of these subspaces. They are of the form \FF=g
(H^2\ominus u H^2) with gβH2 and u inner, u(0)=0. A very particular
fact is that the operator of multiplication by g acts as an isometry on
H2βuH2. Sarason obtained a characterization of the functions g
which act isometrically on H2βuH2. Hayashi obtained the link between
the symbol \phii of a Toeplitz operator and the functions g and u to
ensure that a given subspace \FF=gK_u is the kernel of T_\phii. Chalendar,
Chevrot and Partington studied the nearly Sβ-invariant subspaces for
vector-valued functions. In this paper, we investigate the generalization of
Sarason's and Hayashi's results in the vector-valued context.Comment: 20 pages, accepted by Integral Equations and Operator Theor