In this paper we propose an approach for solving systems of nonlinear equations without computing function derivatives. Motivated by the

application area of tomographic absorption spectroscopy, which is a highly-nonlinear problem with variables coupling, we consider a

situation where straightforward translation to a fixed point problem is not possible because the operators that represent the relevant

systems of nonlinear equations are not self-mappings, i.e., they operate between spaces of different dimensions.

To overcome this difficulty we suggest an "alternating common fixed points algorithm'' that acts alternatingly on the different vector variables. This approach translates the original problem to a common fixed point problem for which iterative algorithms are abound and

exhibits a viable alternative to translation to an optimization problem, which usually requires derivatives information. However, to apply

any of these iterative algorithms requires to ascertain the conditions that appear in their convergence theorems.

To circumvent the need to verify conditions for convergence, we propose and motivate a derivative-free algorithm that better suits the tomographic absorption spectroscopy problem at hand and is even further improved by applying to it the superiorization approach. This is presented along with experimental results that demonstrate our approach.

## Citation

Accepted for publication in the journal: Communications in Optimization Theory