The inverse approach to Dirac-type systems based on the $A$-function concept

The principal objective in this paper is a new inverse approach to general Dirac-type systems modeled after B. Simon’s 1999 inverse approach to half-line Schr"odinger operators. In particular, we derive the so-called A-equation associated to Dirac-type systems and, given a fundamental positivity condition, we prove that this integro-differential equation for A is uniquely solvable. We show how to recover the matrix-valued potential coefficient from A. This approach is also applicable to matrix-valued Schr"odinger operators on a half-line.