Partial differential equations, property C for

Property $C$ stands for "completeness" of the set of products of solutions to homogeneous linear partial differential equations. It was introduced in [a1] and used in [a2], [a3], [a4], [a5], [a6], [a7], [a8], [a9], [a10], [a11], [a12], [a13] as a powerful tool for proving uniqueness results for many multi-dimensional inverse problems, in particular, inverse scattering problems (cf. also Inverse scattering, multi-dimensional case).

Let $D$ be a bounded domain in $\textbf{R}^n$, $n\geq 2$, let $L_mu(x):=\sum^{J}_{|j|=0}a_{jm}(x)D^ju(x)$, where $j$ is a multi-index, $D^ju=\partial^{|j|}u/\partial x_1^{j_{1}}...\partial x_n^{j_{n}}$, derivatives being understood in the distributional sense, the $a_{jm}(x)$, $m=1,2$, are certain $L^{\infty}(D)$ functions, $N_m:=\{w:L_mw=0\text{in}D\}$ is the null-space of the formal differential operator $L_m$, and the equation $L_mw=0$ is understood in the distributional sense.

Consider the subsets $\widetilde{N}_1\in N_2$ and $\widetilde{N}_2\in N_n$ for which the products $w_1w_2$ are defined, $w_1\in\widetilde{N}_1$, $w_2\in\widetilde{N}_2$.

The pair $\{L_1,L_2\}$ has property $C_p$ if and only if the set $\{w_1w_2\}_{\forall w_{m}\in\widetilde{N}_{m}}$ is total (complete) in $L^p(D)$, ($p\geq 1$ is fixed), that is, if $f(x)\in L^p(D)$ and

\begin{equation}\int_{D}f(x)w_1(x)w_2(x)dx=0,\\\forall w_1\in\widetilde{N}_1,\forall w_2\in\widetilde{N}_2,\end{equation}

then $f(x)\equiv 0$.

By property $C$ one often means property $C_2$ or $C_p$ with any fixed $p\geq1$.

Is property $C$ generic for a pair of formal partial differential operators $L_1$ and $L_2$?

For the operators with constant coefficients, a necessary and sufficient condition is given in [a10] for a pair $\{L_1,L_2\}$ to have property $C$. For such operators it turns out that property $C$ is generic and holds or fails to hold simultaneously for all $p\in[1,\infty)$: Assume $a_{jm}(x)=a_{jm}=\text{const}$. Denote $L_m(z):=\sum^{J}_{|j|=0}a_{jm}z^j$, $z\in\textbf{C}^n$. Note that , .

Therefore if and only if .

Define the algebraic varieties (cf. also Algebraic variety)

One says that is transversal to , and writes , if and only if there exist a point and a point such that the tangent space to (in ) at the point and the tangent space to at the point are transversal (cf. Transversality).

The following result is proved in [a1]: The pair of formal partial differential operators with constant coefficients has property if and only if .

Thus, property fails to hold for a pair of formal differential operators with constant coefficients if and only if the variety is a union of parallel hyperplanes in .

Therefore, property for partial differential operators with constant coefficients is generic.

If and the pair has property , then one says that has property .

Examples.

Let , . Then . It is easy to check that there are points and at which the tangent hyperplanes to are not parallel. Thus has property . This means that the set of products of harmonic functions in a bounded domain is complete in , (cf. also Harmonic function). Similarly one checks that the operators

have property .

Numerous applications of property to inverse problems can be found in [a1].

Property holds for a pair of Schrödinger operators with potentials , , where is the set of functions with compact support{} (cf. also Schrödinger equation).

If , , , , is the unit sphere in , are the scattering solutions corresponding to the Schrödinger operators , , , then the set of products , is fixed, is complete in , where is an arbitrary fixed bounded domain [a1]. The set , where is fixed, is total in the set , where is the Sobolev space [a1].

References

[a1]	A.G. Ramm, "Multidimensional inverse scattering problems" , Longman/Wiley (1992)
[a2]	A.G. Ramm, "Scattering by obstacles" , Reidel (1986)
[a3]	A.G. Ramm, "Completeness of the products of solutions to PDE and uniqueness theorems in inverse scattering" Inverse Probl. , 3 (1987) pp. L77–L82
[a4]	A.G. Ramm, "Multidimensional inverse problems and completeness of the products of solutions to PDE" J. Math. Anal. Appl. , 134 : 1 (1988) pp. 211–253 (Also: 139 (1989), 302)
[a5]	A.G. Ramm, "Recovery of the potential from fixed energy scattering data" Inverse Probl. , 4 (1988) pp. 877–886 (Also: 5 (1989), 255)
[a6]	A.G. Ramm, "Multidimensional inverse problems: Uniqueness theorems" Appl. Math. Lett. , 1 : 4 (1988) pp. 377–380
[a7]	A.G. Ramm, "Multidimensional inverse scattering problems and completeness of the products of solutions to homogeneous PDE" Z. Angew. Math. Mech. , 69 : 4 (1989) pp. T13–T22
[a8]	A.G. Ramm, "Property C and uniqueness theorems for multidimensional inverse spectral problem" Appl. Math. Lett. , 3 (1990) pp. 57–60
[a9]	A.G. Ramm, "Completeness of the products of solutions of PDE and inverse problems" Inverse Probl. , 6 (1990) pp. 643–664
[a10]	A.G. Ramm, "Necessary and sufficient condition for a PDE to have property C" J. Math. Anal. Appl. , 156 (1991) pp. 505–509
[a11]	A.G. Ramm, "Property C and inverse problems" , ICM-90 Satellite Conf. Proc. Inverse Problems in Engineering Sci. , Springer (1991) pp. 139–144
[a12]	A.G. Ramm, "Stability estimates in inverse scattering" Acta Applic. Math. , 28 : 1 (1992) pp. 1–42
[a13]	A.G. Ramm, "Stability of solutions to inverse scattering problems with fixed-energy data" Rend. Sem. Mat. e Fisico (2001) pp. 135–211