Let S(Rn)\documentclass[12pt]{minimal}
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\begin{document}$$\mathcal {S}(\mathbb {R}^n)$$\end{document} be the Schwartz space and S′(Rn)\documentclass[12pt]{minimal}
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\begin{document}$$\mathcal {S'}(\mathbb {R}^n)$$\end{document} be the space of tempered distributions on Rn\documentclass[12pt]{minimal}
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\begin{document}$$\mathbb {R}^n$$\end{document}. In this article, we prove that if H⊆S′(Rn)\documentclass[12pt]{minimal}
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\begin{document}$$\mathcal {H} \subseteq \mathcal {S'}(\mathbb {R}^n)$$\end{document} is a non-zero Hilbert space of tempered distributions which is translation and modulation invariant such that |(f,g)|≤C‖f‖H\documentclass[12pt]{minimal}
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\begin{document}$$\begin{aligned} |(f,g)| \le C \Vert f\Vert _{\mathcal {H}} \end{aligned}$$\end{document}for some C>0\documentclass[12pt]{minimal}
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\begin{document}$$C>0$$\end{document} and for all f∈H\documentclass[12pt]{minimal}
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\begin{document}$$f\in \mathcal {H}$$\end{document}, then H=L2(Rn)\documentclass[12pt]{minimal}
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\begin{document}$$\mathcal {H}=L^2(\mathbb {R}^n)$$\end{document}, where g(x)=e-x2\documentclass[12pt]{minimal}
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\begin{document}$$g(x) = e^{-x^2}$$\end{document} for all x∈Rn\documentclass[12pt]{minimal}
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\begin{document}$$x\in \mathbb {R}^n$$\end{document} and (·,·)\documentclass[12pt]{minimal}
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\begin{document}$$(\cdot , \cdot )$$\end{document} denotes the standard duality pairing between S′(Rn)\documentclass[12pt]{minimal}
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\begin{document}$$\mathcal {S'}(\mathbb {R}^n)$$\end{document} and S(Rn)\documentclass[12pt]{minimal}
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\begin{document}$$\mathcal {S}(\mathbb {R}^n)$$\end{document} with respect to which (S(Rn))∗=S′(Rn)\documentclass[12pt]{minimal}
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\begin{document}$$(\mathcal {S}(\mathbb {R}^n))^*=\mathcal {S'}(\mathbb {R}^n)$$\end{document}.
