code: public static void invertLower( double L[] , int m ) {
          for( int i = 0; i < m; i++ ) {
              double L_ii = L[ i*m + i ];
              for( int j = 0; j < i; j++ ) {
                  double val = 0;
                  for( int k = j; k < i; k++ ) {
                      val += L[ i*m + k] * L[ k*m + j ];
                  }
                  L[ i*m + j ] = -val / L_ii;
              }
              L[ i*m + i ] =  1.0 / L_ii;
          }
      }
docs: <p>
  Inverts a square lower triangular matrix:  L = L<sup>-1</sup>
  </p>
  
  
  @param L
  @param m