Feshchenko I. S.
Ukr. Mat. Zh. - 2011. - 63, № 10. - pp. 1381-1425
We give necessary and sufficient conditions for the sum of subspaces $H_1,..., H_n,, \quad n \geq 2,$ of a Hilbert space $H$ to be a subspace and present various properties of $n$-tuples of subspaces with closed sum.