  As the most successful example of a hybrid tomographic technique,
photoacoustic tomography is based on generating acoustic waves inside an object
of interest by stimulating electromagnetic waves. This acoustic wave is
measured outside the object and converted into a diagnostic image. One
mathematical problem is determining the initial function from the measured
data. The initial function describes the spatial distribution of energy
absorption, and the acoustic wave satisfies the wave equation with variable
speed. In this article, we consider two types of problems: inverse problem with
Robin boundary condition and inverse problem with Dirichlet boundary condition.
We define two wave-forward operators that assign the solution of the wave
equation based on the initial function to a given function and provide their
inversions.
