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Let $f, g\in L^{2},$ by Plancherel’s theorem, we have

$$\langle f, g \rangle= \langle \hat{f}, \hat{g} \rangle.$$

My Question is:

Is it true that:

$$\langle f, g \rangle= \langle \hat{f}, \hat{g} \rangle$$

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for $f\in \mathcal{S’}(\mathbb R^d)$ (tempered distribution) and $g\in \mathcal{S}(\mathbb R^d)$(Schwartz space)? If yes, how to justify.

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One has to be careful with complex conjugation here. The claimed identity is

$$

\int f \bar g = \int \hat f \bar {\hat g}

\tag1$$

Conjugating the (unitary) Fourier transform gives the inverse Fourier transform of conjugation. So,

$$

\int \hat f\bar {\hat g} = \int \hat f \check {\bar g}

\tag2$$

The right hand side is the value of distribution $\hat f$ on test function $\check {\bar g} $. By the definition of $\hat f$, this is computed by passing the hat to the test function, which cancels out the inverse hat:

$$

\int \hat f \check {\bar g} = \int f \bar g

\tag3$$

I think the answer lies in this characterization:

Explicit characterization of dual of $H^1$

knowing, in particular, that $\mathcal{F} : \mathcal{S}'(\mathbb{R}^n) \longrightarrow \mathcal{S}'(\mathbb{R}^n)$ is an isomorphism, extension of $\mathcal{F}:L^2(\mathbb{R}^n) \longrightarrow L^2(\mathbb{R}^n)$.

TrialAndError did you notice that there is no scalar product in the space of tempered distributions. But you can understand this, in the sense of the characterization of dual space $H^s({\mathbb{R}^n})^*$, or simply of $H^s(\mathbb{R}^n)$.

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