# Verifying fundamental solution groups for lossless wave equations via stationary action and optimal control

## TL;DR

This paper introduces a novel method for representing fundamental solution groups of lossless wave equations using stationary action and optimal control, enabling approximation over long time horizons with convergence guarantees.

## Contribution

It develops a new approach connecting stationary action with optimal control to approximate wave equation solutions over extended periods.

## Key findings

- Approximate solution groups converge strongly to exact groups.
- Representation via short horizon optimal control is effective.
- Method applied successfully to boundary value problems.

## Abstract

A representation of a fundamental solution group for a class of wave equations is constructed by exploiting connections between stationary action and optimal control. By using a Yosida approximation of the associated generator, an approximation of the group of interest is represented for sufficiently short time horizons via an idempotent convolution kernel that describes all possible solutions of a corresponding short time horizon optimal control problem. It is shown that this representation of the approximate group can be extended to arbitrary longer horizons via a concatenation of such short horizon optimal control problems, provided that the associated initial and terminal conditions employed in concatenating trajectories are determined via a stationarity rather than an optimality based condition. The long horizon approximate group obtained is shown to converge strongly to the exact group of interest, under reasonable conditions. The construction is illustrated by its application to the solution of a two point boundary value problem.