Spectral Theorem for quaternionic normal operators: Multiplication form
Let $\mathcal{H}$ be a right quaternionic Hilbert space and let $T$ be a quaternionic normal operator with the domain $\mathcal{D}(T) \subset \mathcal{H}$. Then for a fixed unit imaginary quaternion $m$, there exists a Hilbert basis $\mathcal{N}_{m}$ of $\mathcal{H}$, a measure space $(Ω, μ)$, a unitary operator $U \colon \mathcal{H} \to L^{2}(Ω; \mathbb{H}; μ)$ and a $μ$ - measurable function $ϕ\colon Ω\to \mathbb{C}_m$ (here $\mathbb{C}_{m} = \{α+ m β; \;α, β\in \mathbb{R}\}$) such that \[ Tx = U^{*}M_ϕUx, \; \mbox{for all}\; x\in \mathcal{D}(T), \] where $M_ϕ$ is the multiplication operator on $L^{2}(Ω; \mathbb{H}; μ)$ induced by $ϕ$ with $ U(\mathcal{D}(T)) \subseteq \mathcal{D}(M_ϕ)$. In the process, we prove that every complex Hilbert space is a slice Hilbert space. We establish these results by reducing it to the complex case then lift it to the quaternionic case.