 which involves the solution of triangular systems of
  equations that are constructed recursively and which may be nearly
  singular if A and -B have close eigenvalues. If near singularity
  is detected, then the routine returns with the Error Indicator
  (INFO) set to 1.

</PRE>
<A name="References"><B><FONT SIZE="+1">References</FONT></B></A>
<PRE>
  [1] Golub, G.H., Nash, S. and Van Loan, C.F.
      A Hessenberg-Schur method for the problem AX + XB = C.
      IEEE Trans. Auto. Contr., AC-24, pp. 909-913