title: ComputeThinU VS ComputeFullU in Eigen JacobiSVD
body: <p>I'm trying to solve a linear equation using JacobiSVD in Eigen. There are two decomposition options for JacobiSVD: ComputeThinU and ComputeFullU. Their meanings are explained as</p>
  
  <ul>
  <li>ComputeFullU: in JacobiSVD to indicate that the square matrix U is to be computed.</li>
  <li>ComputeThinU：in JacobiSVD to indicate that the thin matrix U is to be computed.</li>
  </ul>
  
  <p>What does the "thin matrix" mean? And how do I determine which option to use?</p>