The invariant subspace problem is one of the longstanding open problem in the field of functional analysis and operator theory. It is due to J. von Neumann (in 1932) and is stated as: Does every operator have a nontrivial invariant subspace? An invariant subspace for an operator means a subspace such that the image of the subspace under the operator returns to the subspace. This problem remains still open for infinite-dimensional separable Hilbert spaces. In this talk we consider the reason why so much attention to the invariant subspace problem and discuss the invariant subspaces of the shifts. In the context of this problem, we come across G.H. Hardy and A. Beurling.
