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I want to prove the following: (Here, $W^{2,2}$ is a Sobolev space as defined in Evans, chapter 5; $S$ is a Schwartz space; and if $A$ is a distribution and $a$ a function, then $\langle A, a\rangle$ means $A(a)$).

Theorem. Let $\newcommand{\C}{\mathbb C}\newcommand{\R}{\mathbb R}C\in]0,\infty[$. For every $f\in L^2 (\mathbb R;\C)$ there exists a $g\in (L^2 \cap W^{2,2}_{\text{loc}})(\R;\C)$ such that (in the weak sense) \begin{equation} -g'' + C^2 g = f \end{equation} and an explicit solution is given by \begin{equation} g(t) = \frac{1}{2C} \int_{\R} e^{-C|t-\tau|} f (\tau)\,\mathrm d\tau. \end{equation}


My attempt. (Skip to the bottom for my question)

Consider the tempered distribution \begin{equation*}\begin{split} \mathscr Z: S(\R;\C)&\to\C, \\ \phi&\mapsto\int_\R e^{-{C\lvert t\rvert}} \phi(t)\,\mathrm dt = \int_0^\infty e^{-Ct}\phi(t)\,\mathrm dt+\int_{-\infty}^0 e^{Ct} \phi(t)\,\mathrm dt. \end{split}\end{equation*} Then we have, for all $\phi\in S(\R;\C)$, \begin{equation*} \langle\mathscr Z',\phi\rangle=-\langle\mathscr Z,\phi'\rangle=\int_0^\infty e^{-Ct}\phi'(t)\,\mathrm dt+\int_{-\infty}^0 e^{Ct} \phi'(t)\,\mathrm dt. \end{equation*} Integrating both terms by parts, where the exponential term gets differentiated and $\phi'$ gets integrated, we get \begin{equation*}\begin{split} -\langle\mathscr Z',\phi\rangle &= \left[e^{-Ct}\phi(t)\right]^\infty_0+C\int_0^\infty e^{-Ct}\phi(t)\,\mathrm dt+\left[e^{Ct}\phi(t)\right]^0_{-\infty}-C\int_{-\infty}^0 e^{Ct}\phi(t)\,\mathrm dt \\ &= C\int_0^\infty e^{-Ct}\phi(t)\,\mathrm dt-C\int_{-\infty}^0 e^{Ct}\phi(t)\,\mathrm dt. \end{split}\end{equation*} Integrating both terms by parts in the same way, we get (where $\delta_0$ is the Dirac distribution at $0$) \begin{equation*}\begin{split} \langle\mathscr Z'',\phi\rangle &= \langle-\mathscr Z',\phi'\rangle \\ &= C\left[e^{-Ct}\phi(t)\right]^\infty_0+C^2\int_0^\infty e^{-Ct}\phi(t)\,\mathrm dt - \left(C \left[e^{Ct}\phi(t)\right]_{-\infty}^0-C^2\int_{-\infty}^0 e^{Ct}\phi(t)\,\mathrm dt\right) \\ &= C^2 \langle\mathscr Z,\phi\rangle-2C\phi(0) = C^2 \langle\mathscr Z,\phi\rangle-2 C\langle{\delta_0,\phi}\rangle. \end{split}\end{equation*}


Now I would like to finish by writing

$$g = \frac{\mathscr Z* f}{2C}$$ and therefore

$$-g''+C^2 g = \frac{-(\mathscr Z'' * f)+C^2 (\mathscr Z* f)}{2C} = \frac{-((C^2\mathscr Z-2C\delta_0)*f)+C^2 (\mathscr Z* f)}{2C}=\delta_0*f = f.$$

My question: However, to do this, I formally use $$ (A*a)'=(A'*a)$$ when $A$ is a distribution. Is there some result that justifies this? And furthermore, does this result also imply that $g$, defined as the convolution of $f$ with $\mathscr Z$, can be written as a $W^{2,2}_{\text{loc}}$ function?

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  • $\begingroup$ do u know how $A'$ is defined? $\endgroup$ Aug 7 at 21:04
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    $\begingroup$ You still have the usual (existence and uniqueness) theory available also for ODEs in this more general interpretation; this is discussed in many books, for example Coddington-Levinson. Your formula is then the variation-of-constants formula for the solutions of an inhomogeneous linear ODE. $\endgroup$ Aug 7 at 22:00
  • $\begingroup$ @ChristianRemling Thank you! I will check out the Coddington-Levinson book that you mentioned in the next days $\endgroup$ Aug 7 at 23:24
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    $\begingroup$ @mathworker21 (I do not know of a sensible way to define the convolution $T*f$ if $T$ is any tempered distribution and $f$ is any $L^2$-function.) $\endgroup$ Aug 7 at 23:32
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    $\begingroup$ @mathworker21 ahh I apologize for getting a bit pissed then 😅 $\endgroup$ Aug 8 at 21:51
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The last step is formally justified by 15.8, differentiation property of José Sebastião e Silva's "Integrals and orders of growth of distributions." (The paper is currently available here.)

More precisely: $\mathscr K$ is defined by a continuous function. Also, by Lemma 8.2 of Haim Brezis' Functional Analysis, Sobolev Spaces and Partial Differential Equations (2010), we have, in the weak sense, since $f\in L^2\subset L^1_{\text{loc}}$, $F'=f$ where $F(x):=\int_0^x f$ is continuous.

Therefore, one can indeed apply 15.8 mentioned above.

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