    June  2014, 3(2): 247-256. doi: 10.3934/eect.2014.3.247

## Non-smooth unobservable states in control problem for the wave equation in $\mathbb{R}^3$

 1 Saint-Petersburg Department of the Steklov Mathematical Institute, Saint-Petersburg State University, Russian Federation, Russian Federation

Received  November 2013 Revised  April 2014 Published  May 2014

The paper deals with a dynamical system \begin{align*} &u_{tt}-\Delta u=0, \qquad (x,t) \in {\mathbb R}^3 \times (-\infty,0) \\ &u \mid_{|x|<-t} =0 , \qquad t<0\\ &\lim_{s \to \infty} su((s+\tau)\omega,-s)=f(\tau,\omega), \qquad (\tau,\omega) \in [0,\infty)\times S^2\,, \end{align*} where $u=u^f(x,t)$ is a solution ( wave), $f \in {\mathcal F}:=L_2\left([0,\infty);L_2\left(S^2\right)\right)$ is a control. For the reachable sets ${\mathcal U}^\xi:=\{u^f(\cdot,-\xi)\,|\,\, f \in {\mathcal F}\}\,\,(\xi\geq 0)$, the embedding ${\mathcal U}^\xi \subset {\mathcal H}^\xi:=\{y \in L_2({\mathbb R}^3)\,|\,\,\,y|_{|x|<\xi}=0\}$ holds, whereas the subspaces ${\mathcal D}^\xi:={\mathcal H}^\xi \ominus {\mathcal U}^\xi$ of unreachable ( unobservable) states are nonzero for $\xi> 0$. There was a conjecture motivated by some geometrical optics arguments that the elements of ${\mathcal D}^\xi$ are $C^\infty$-smooth with respect to $|x|$. We provide rather unexpected counterexamples of $h\in {\mathcal D}^\xi$ with ${\rm sing\,supp\,}h \subset \{x\in{\mathbb R}^3|\,\,|x|=\xi_0>\xi\}$.
Citation: Mikhail I. Belishev, Aleksei F. Vakulenko. Non-smooth unobservable states in control problem for the wave equation in $\mathbb{R}^3$. Evolution Equations & Control Theory, 2014, 3 (2) : 247-256. doi: 10.3934/eect.2014.3.247
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##### References:
  S. A. Avdonin, M. I. Belishev and S. I. Ivanov, Controllability in the filled domain for the wave equation with a singular boundary control, (in Russian) Zap. Nauch. Semin. POMI, 210 (1994), 7-21; English translation in J. Sov. Math., 83 (1997), 165-174. doi: 10.1007/BF02405808.  Google Scholar  M. I. Belishev, Recent progress in the boundary control method, Invers Problems, 23 (2007), R1-R67. doi: 10.1088/0266-5611/23/5/R01.  Google Scholar  M. I. Belishev and A. F. Vakulenko, On a control problem for the wave equation in $\mathbbR^3$, (in Russian) Zap. Nauch. Semin. POMI, 332 (2006), 19-37; English translation in J. Math. Sci., 142 (2007), 2528-2539. doi: 10.1007/s10958-007-0140-3.  Google Scholar  M. I. Belishev and A. F. Vakulenko, Reachable and unreachable sets in the scattering problem for the acoustical equation in $\mathbbR^3$, SIAM J. Math. Analysis, 39 (2008), 1821-1850. doi: 10.1137/060678877.  Google Scholar  M. I. Belishev and A. F. Vakulenko, $s$-points in three-dimensional acoustical scattering, SIAM J. Math. Analysis, 42 (2010), 2703-2720. doi: 10.1137/090781486.  Google Scholar  S. Helgason, The Radon Transform, Birhausser, Boston, Basel, Stuttgart, 1999. doi: 10.1007/978-1-4757-1463-0.  Google Scholar  L. Hörmander, The Analysis of Linear Partial Differential Operators I, Springer-Verlag, Berlin, Heidelberg, New-York, Tokyo, 1983. Google Scholar  M. Ikawa, Hyperbolic Partial Differential Equations and Wave Fenomena, Translated from the 1997 Japanese original by Bohdan I. Kurpita, Translations of Mathematical Monographs, 189, Iwanami Series in Modern Mathematics. American Mathematical Society, Providence, RI, 2000. Google Scholar  I. Lasiecka, J-L. Lions and R. Triggiani, Non homogeneous boundary value problems for second order hyperbolic operators, J. Math. Pures Appl, 65 (1986), 149-192. Google Scholar  P. Lax and R. Phillips, Scattering Theory, Academic Press, New-York-London, 1967. Google Scholar  D. L. Russell, Boundary value control theory of the higher-dimensional wave equation, SIAM J. Control, 9 (1971), 29-42. doi: 10.1137/0309004.  Google Scholar
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