
      <pre>
        sum ( 1 <= i <= nd ) w(i) * phi(||xd(j)-xd(i)||) = fd(j)
      </pre>
    </p>

    <p>
      Four families of radial basis functions are provided. 
      <ul>
        <li>
          phi1(r) = sqrt ( r^2 + r0^2 ) (multiquadric)
        </li>
        <li>
          phi2(r) = 1 / sqrt ( r^2 + r0^2 ) (inverse multiquadric)
        </li>
        <li>
          phi3(r) = r^2 * log ( r / r0 )   (thin plate spline)
        </li>
        <li>
          phi4(r) = exp ( -0.5 r^2 / r0^2 )   (gauss