E. Zikkos

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math.FA2026

Hardy Subspaces with Sparse Fourier Spectrum and Muntz Space

Elias Zikkos

Let $Λ = {λ_{n}}_{n = 1}^{\infty} \subset N$ with $λ_{n}$ strictly increasing and such that $\sum_{n = 1}^{\infty} λ_{n}^{- 1} < \infty$ . We show that a Hardy subspace $H^2 (\mathbb{D}, Λ…

math.FA2026

The Gaussian Gabor system at the critical density is a weighted lower semi frame

Elias Zikkos

We show that the Gaussian Gabor system at the critical density is a weighted lower semi frame, thus resolving an open question. This complements recent results showing that the sys…

math.FA2022

Characterizing Riesz Bases via Biorthogonal Riesz-Fischer sequences

Elias Zikkos

In this note we prove that if two Riesz-Fischer sequences in a separable Hilbert space $H$ are biorthogonal and one of them is complete in $H$ , then both sequences are Riesz bases…