09-107 P. Exner, H. Neidhardt
Trotter-Kato product formula for unitary groups (296K, pdf) Jul 7, 09
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Abstract. Let $A$ and $B$ be non-negative self-adjoint operators in a separable Hilbert space such that its form sum $C$ is densely defined. It is shown that the Trotter product formula holds for imaginary times in the $L^2$-norm, that is, one has % % \begin{displaymath} \lim_{n\to+\infty}\int^T_0 \left\|\left(e^{-itA/n}e^{-itB/n}\right)^nh - e^{-itC}h\right\|^2dt = 0 \end{displaymath} % % for any element $h$ of the Hilbert space and any $T > 0$. The result remains true for the Trotter-Kato product formula % % \begin{displaymath} \lim_{n\to+\infty}\int^T_0 \left\|\left(f(itA/n)g(itB/n)\right)^nh - e^{-itC}h\right\|^2dt = 0 \end{displaymath} % % where $f(\cdot)$ and $g(\cdot)$ are so-called holomorphic Kato functions; we also derive a canonical representation for any function of this class.

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