On biorthogonal systems whose functionals are finitely supported

TL;DR

The paper demonstrates the consistency of the existence of certain biorthogonal systems in Banach spaces of continuous functions on specially constructed compact spaces, highlighting differences in measure support sizes and their implications.

Contribution

It constructs compact spaces where biorthogonal systems with atomic measures of specific support sizes exist or do not, revealing new structural properties of these spaces.

Findings

Existence of biorthogonal systems with supports of size 2n

Non-existence of certain biorthogonal systems with supports of size 2n-1

Hereditary density varies with power of the space

Abstract

We show that for each natural $n>1$ it is consistent that there is a compact Hausdorff space $K_{2n}$ such that in $C(K_{2n})$ there is no uncountable (semi)biorthogonal sequence $(f_\xi,\mu_\xi)_{\xi\in \omega_1}$ where $\mu_\xi$'s are atomic measures with supports consisting of at most $2n-1$ points of $K_{2n}$, but there are biorthogonal systems $(f_\xi,\mu_\xi)_{\xi\in \omega_1}$ where $\mu_\xi$'s are atomic measures with supports consisting of $2n$ points. This complements a result of Todorcevic that it is consistent that each nonseparable Banach space $C(K)$ has an uncountable biorthogonal system where the functionals are measures of the form $\delta_{x_\xi}-\delta_{y_\xi}$ for $\xi<\omega_1$ and $x_\xi,y_\xi\in K$. It also follows that it is consistent that the irredundance of the Boolean algebra $Clop(K)$ or the Banach algebra $C(K)$ for $K$ totally disconnected can be strictly smaller than the sizes of biorthogonal systems in $C(K)$. The compact spaces exhibit an interesting behaviour with respect to known cardinal functions: the hereditary density of the powers $K_{2n}^k$ is countable up to $k=n$ and it is uncountable (even the spread is uncountable) for $k>n$.