Classification of Bounded Sesquilinear Forms

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Theorem

Let $H, K$ be Hilbert spaces over $\Bbb F \in \left\{{\R, \C}\right\}$.

Let $u: H \times K \to \Bbb F$ be a bounded sesquilinear form with bound $M$.


Then there exist unique bounded linear transformations $A \in B \left({H, K}\right), B \in B \left({K, H}\right)$ such that:

$\forall h \in H, k \in K: u \left({h, k}\right) = \left\langle{Ah, k}\right\rangle_K = \left\langle{h, Bk}\right\rangle_H$

Furthermore, $\left\Vert{A}\right\Vert, \left\Vert{B}\right\Vert \le M$.


Proof


Also see


Sources