= Hereditarily indecomposable Banach space
An infinite-dimensional Banach space is hereditarily indecomposable if no closed infinite-dimensional subspace is a topological direct sum of two infinite-dimensional closed subspaces. Equivalently, for any two infinite-dimensional closed subspaces $U,V$ and any $\varepsilon>0$, some unit vectors $u\in U,v\in V$ satisfy $\|u-v\|<\varepsilon$. A positive lower bound on such distances would make the addition map from $U\oplus V$ an isomorphism onto a closed subspace.
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