Abstract
Given a domain $D$ in $\\mathbb{C}^n$ and a compact subset $K$ of $D$, the set $\\mathcal{A}K^D$ of all restrictions of functions holomorphic on $D$ the modulus of which is bounded by $1$ is a compact subset of the Banach space $C(K)$ of continuous functions on $K$. The sequence $(d_m(\\mathcal{A}\_K^D)){m\\in \\mathbb{N}}$ of Kolmogorov $m$-widths of $\\mathcal{A}\_K^D$ provides a measure of the degree of compactness of the set $\\mathcal{A}\_K^D$ in $C(K)$ and the study of its asymptotics has a long history, essentially going back to Kolmogorov’s work on $\\epsilon$-entropy of compact sets in the 1950s. In the 1980s Zakharyuta showed that for suitable $D$ and $K$ the asymptotics $$ \\lim\_{m\\to \\infty}\\frac{- \\log d_m(\\mathcal{A}\_K^D)}{m^{1/n}} = 2\\pi \\bigg( \\frac{n!}{C(K,D)} \\bigg) ^{1/n}, $$ where $C(K,D)$ is the Bedford–Taylor relative capacity of $K$ in $D$, is implied by a conjecture, now known as Zakharyuta's Conjecture, concerning the approximability of the regularised relative extremal function of $K$ and $D$ by certain pluricomplex Green functions. Zakharyuta’s Conjecture was proved by Nivoche in 2004 thus settling (1) at the same time. We shall give a new proof of the asymptotics (1) for $D$ strictly hyperconvex and $K$ non-pluripolar which does not rely on Zakharyuta’s Conjecture. Instead we proceed more directly by a two-pronged approach establishing sharp upper and lower bounds for the Kolmogorov widths. The lower bounds follow from concentration results of independent interest for the eigenvalues of a certain family of Toeplitz operators, while the upper bounds follow from an application of the Bergman–Weil formula together with an exhaustion procedure by special holomorphic polyhedra.
Highlights
The bounds for the Kolmogorov numbers for the natural embedding H2(D) → L2(K) for a bounded pseudoconvex domain D in Cn and K a compact subset of D with positive Lebesgue measure were obtained under the additional assumption that φ be strictly psh and smooth on D
If D is hyperconvex and K is a non-pluripolar compact subset of D, which we do not assume to be holomorphically convex in D, the relative extremal function uK,D is lower semicontinuous on D, see [Kli[91], Corollary 4.5.11]
Our strategy for determining sharp upper bounds for the Kolmogorov widths is based on the Bergman-Weil formula coupled with an approximation argument
Summary
If D is a hyperconvex domain in Cn containing a compact subset K, u∗K,D is maximal on D \K and (ddcu∗K,D)n = 0 on D \K In this case, the complex Monge-Ampere operator (ddcu∗K,D)n is well defined and turns out to be a positive measure supported on K [Kli[91], Section 4.5]. In order to solve Kolmogorov’s problem, it is sufficient to prove that Zakharyuta’s Conjecture is true, as was shown in [SZ76] for n = 1 and in [Zak85] (see [Zak[09], Zak11a]) for n > 1, provided that K be regular in D with non-zero Lebesgue measure, and that the domain D be strictly hyperconvex, a rather natural notion that is defined as follows. We note that in the particular case where X and Y are Hilbert spaces, the Kolmogorov numbers, Gelfand numbers and approximation numbers coincide (see, for example, [Pie[87], Theorem 2.11.9])
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