Definition:Partition of Unity (Hilbert Space)
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This page is about Partition of Unity in the context of Hilbert Space. For other uses, see Partition of Unity.
Definition
Let $H$ be a Hilbert space.
A partition of unity or partition of identity on $H$ is a family $\family {P_i}_{i \mathop \in I}$ of projections, subject to:
- If $i \ne j$, then $P_i P_j = P_j P_i = 0$
- $\vee \set {\Img {P_i}: i \in I} = H$, where $\vee$ signifies closed linear span
One may encounter the notations $1 = \sum_i P_i$ and $1 = \bigoplus_i P_i$.
Here, $1$ signifies the identity operator on $H$.
Sources
- 1990: John B. Conway: A Course in Functional Analysis (2nd ed.) ... (previous) ... (next) $\text {II}.7.2$