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.

When $I$ is finite, by Closed Linear Subspaces Closed under Setwise Addition, have that:


 * $\displaystyle \bigoplus_{i \in I} M_i = \sum_{i \in I} M_i$, where $\displaystyle \sum$ signifies setwise addition.

Also see

 * Hilbert Space Direct Sum, a broad generalization