ONT></B>
<PRE>
  To solve for X the discrete-time Sylvester equation

     X + AXB = C,

  where A, B, C and X are general N-by-N, M-by-M, N-by-M and
  N-by-M matrices respectively. A Hessenberg-Schur method, which
  reduces A to upper Hessenberg form, H = U'AU, and B' to real
  Schur form, S = Z'B'Z (with U, Z orthogonal matrices), is used.

</PRE>
<A name="Specification"><B><FONT SIZE="+1">Specification</FONT></B></A>
<PRE>
      SUBROUTINE SB04QD( N, M, A, LDA, B, LDB, C, LDC, Z, LDZ, IWORK,
     $      