# Partial data inverse problems and simultaneous recovery of boundary and coefficients for semilinear elliptic equations

### Matti Lassas

University of Helsinki, Finland### Tony Liimatainen

University of Jyväskylä, Finland### Yi-Hsuan Lin

University of Jyväskylä, Finland, and National Chiao Tung University, Hsinchu, Taiwan### Mikko Salo

University of Jyväskylä, Finland

## Abstract

We study various partial data inverse boundary value problems for the semilinear elliptic equation $Δu+a(x,u)=0$ in a domain in $R_{n}$ by using the higher order linearization technique introduced by Lassas–Liimatainen–Lin–Salo and Feizmohammadi–Oksanen. We show that the Dirichlet-to-Neumann map of the above equation determines the Taylor series of $a(x,z)$ at $z=0$ under general assumptions on $a(x,z)$. The determination of the Taylor series can be done in parallel with the detection of an unknown cavity inside the domain or an unknown part of the boundary of the domain. The method relies on the solution of the linearized partial data Calderón problem by Ferreira–Kenig–Sjöstrand–Uhlmann, and implies the solution of partial data problems for certain semilinear equations $Δu+a(x,u)=0$ also proved by Krupchyk–Uhlmann.

The results that we prove are in contrast to the analogous inverse problems for the linear Schrödinger equation. There recovering an unknown cavity (or part of the boundary) and the potential simultaneously are longstanding open problems, and the solution to the Calderón problem with partial data is known only in special cases when $n≥3$.

## Cite this article

Matti Lassas, Tony Liimatainen, Yi-Hsuan Lin, Mikko Salo, Partial data inverse problems and simultaneous recovery of boundary and coefficients for semilinear elliptic equations. Rev. Mat. Iberoam. 37 (2021), no. 4, pp. 1553–1580

DOI 10.4171/RMI/1242