Definition:Internal Hilbert Space Direct Sum

Definition
Let $H$ be a Hilbert space.

Let $\left\{{M_i : i \in I}\right\}$ be an $I$-indexed collection of pairwise orthogonal subspaces of $H$.

Then the internal (Hilbert space) direct sum of the $M_i$ is their closed linear span $\vee_i M_i$.

It is denoted by $\bigoplus_i M_i$, or $\displaystyle \bigoplus_{i \in I} M_i$ if the set $I$ is to be stressed.