On closed linear subspaces embedded into Banach spaces and their finite-dimensionality
This paper studies a Grothendieck-type finite-dimensionality problem for closed linear subspaces embedded in Banach spaces. Let $S_p^{(q)} \subset L_p(M,dμ)$ be a closed linear subspace of the Banach space $L_p(M,dμ)$ defined with respect to a probability measure $dμ$ on $M$. We prove that if $S_p^{(q)}$ is continuously embedded into $L_q(M,dμ)$ for $q>p$, then its dimension $\dim S_p^{(q)} = N \in \mathbb{N}$ satisfies the estimate \[ \frac{1}{N}\left(\frac{\sqrtπ,Γ!\left(\frac{N+\tilde q}{2}\right)}{Γ!\left(\frac{\tilde q+1}{2}\right)Γ!\left(\frac{N}{2}\right)}\right)^{2/\tilde q}\le K_{p,q(m)}^2, \] where $1/\tilde q + 1/q = 1$, $q = 2 + (p-2)2^m > p$ with $p \neq 2$ and $m \in \mathbb{N}$, and $K_{p,q(m)}>0$ is a bounded constant. We also prove that certain closed linear subspaces of $L_p(M,dμ)$ consisting of continuous functions on $M$ must be finite-dimensional.