when the normal magnetic field is zero I'm running into all sorts of algebraic limits that, if not evaluated correctly, produce divison by zero errors.  if they are evaluated correctly then a few of them, summed together, produce the negative speed of sound squared, to cancel out the final component of the slow wave eigenvector's positive speed of sound squared.  i think i'll try a different set of eigenvectors/rescaling, moving once again to Trangenstein where I started, except that last time I forgot to transform them by the change in state wrt primitive matrix.  I still need to solve the left eigenvectors, most likely numerically.