Structure of sets of bounded sequences with a prescribed number of accumulation points

Abstract: For each vector $x\in \ell^{\infty}$, we can define the non-empty compact set $L_x$ of accumulation points of $x$. Given an infinite subset $A$ of $\mathbb{N}\backslash{1}$, we can therefore investigate under which conditions on $A$, the set $L(A):={x\in \ell^\infty: |L_x|\in A}$ is lineable or even densely lineable. In particular, we show that if $L(A)$ is lineable then there exists $k\ge 1$ such that $A\cap (A-k)$ is infinite and that if $L(A)$ is densely lineable then $A\cap (A-1)$ is infinite. We end up by answering an open question on the existence of a closed non-separable subspace in which each non-zero vector has countably many accumulation points.