The inverse spectral problem for first order systems on the half line
  On the half line $[0,\infty)$ we study first order differential operators of the form $B 1/i d/(dx) + Q(x)$, where $B:=\mat{B_1}{0}{0}{-B_2}$, $B_1,B_2\in M(n,\C)$ are self--adjoint positive definite matrices and $Q:\R_+\to M(2n,\C)$, $\R_+:=[0,\infty)$, is a continuous self-adjoint off-diagonal matrix function.   We determine the self-adjoint boundary conditions for these operators. We prove that for each such boundary value problem there exists a unique matrix spectral function $\sigma$ and a generalized Fourier transform which diagonalizes the corresponding operator in $L^2_{\sigma}(R, C)$.   We give necessary and sufficient conditions for a matrix function $\sigma$ to be the spectral measure of a matrix potential $Q$. Moreover we present a procedure based on a Gelfand-Levitan type equation for the determination of $Q$ from $\sigma $.   Our results generalize earlier results of M. Gasymov and B. Levitan.   We apply our results to show the existence of $2n\times 2n$ Dirac systems with purely absolute continuous, purely singular continuous and purely discrete spectrum of multiplicity $p$, where $1\le p \le n$ is arbitrary. 
Matthias Lesch (Humboldt-University at Berlin) and Mark M. Malamud   (Donetsk)