Kolmogorov Widths on the Sphere via Eigenvalue Estimates for Hölderian Integral Operators

Approximation processes in the reproducing kernel Hilbert space associated to a continuous kernel on the unit sphere S m in the Euclidean space R m + 1 are known to depend upon the Mercer’s expansion of the compact and self-adjoint L 2 ( S m ) -operator associated to the kernel. The estimation of the Kolmogorov n th width of the unit ball of the reproducing kernel Hilbert space in L 2 ( S m ) and the identification of the so-called optimal subspace usually suffice. These Kolmogorov widths can be computed through the eigenvalues of the integral operator associated to the kernel. This paper provides sharp upper bounds for the Kolmogorov widths in the case in which the kernel satisfies an abstract Hölder condition. In particular, we follow the opposite direction usually considered in the literature, that is, we estimate the widths from decay rates for the sequence of eigenvalues of the integral operator.