【正确答案】下面给出的函数dir2par将直接系数和转化为并联系数：
   function [C，B，A]=dir2par(b，a)；
   程序如下：
   /%DIRECT-form tO ARALLEL-form conyersion
   /%-------------------------
   /%[C，B，A]=dir2par(b，a)；
   /%C=Polynomial part when length(b)＞=length(a)
   /%B=K by 2 matrix of real coefficients containing bk's
   /%A=K by 3 matrix of real coefficients containing ak's
   /%b=number polynomial coefficients of DIRECT form
   /%a=denomirnator polynomial coefficients of DIRECT form
   /%
   M=length(b)；N=length(a)：
   [r1，p1，C]=residuez(b，a)；
   p=cplxpair(p1，10000000*eps)；
   I=cplxcomp(1l，p)；
   R=r1(I)：
   K=floor(N/2)；B=zero(K，2)；A=zero(K，3)；
   If K*2==N；/%N even，order of A(z)odd，one factor is first order
   fori=1: 2:N-2
   Brow=r(i: 1: i+1,: );
   Arow=p(i: 1: i+1,: );
   [Brow,Arow]=residuez(r(N-1),p(N-1),[]);
   B(K,: )=[real(Brow) 0]; A(K,: )=[real(Arow) 0];
   else
      for i=1: 2:N-1
   Brow=r(i: 1: i+1. : );
   Arow=p(i: 1: i+1. : );
   [Brow, Arow] = residuez(Brow, Arow, []) ;
   B(fix((i+2)/2) ,: )=real(Brow) ;
   A(fix((i+2)/2) ,: )=real(Arow) ;
   end
   end
   function I = cplxcomp( p1, p2 )
   /%I= cplcomp ( p1, p2 )
   /%Compares two complex pairs which contain the same scalar elements
   /%but(possibly) at different indices. This routine shoule be
   /%used after CPLXPAIR routine for rearranging pole vector and its corresponding residue /%vector.
   corresponding residue vector.
   /%P2 = cplxpairz( p1 )
   I=[];
   for j=1: 1: length(p2)
   If(abs(pl(i) - p2(j) ) ＜0.0001
   I[I, i] ;
      end
     end
   end
   I=I';
   ＞＞b=[1 -3 11 -27 18];
   ＞＞a=[16 12 2 -4 -I];
   ＞＞[C, B, A] = dir2par(b, a)
   C=
      -18
   B=
     10.500  -3.9500
     28.1125  -13.3625
   A=
     1.0000     1.0000     0.5000
     1.0000     -0.2500    -0.1250
   下面给出的函数dir2cas将直接系数化为级联系数
   /%function[b0, B, A] = dir2cas( b, a) ;
   /%DIRECT-form to CASCADE- form conversion(cplxpair version)
   /%------------------
   /%[b0, B, A] = dir2cas ( b, a)
   /%b0 = gain coefficient
   /%B=K by 3 matrix of real coefficients containing bk's
   /%A=K by 3 matrix of real coefficients containing ak's
   /%b= numerator polynomial coefficients of DIRECT form
   /%a= denominator polynomial coefficients of DIRECT form
   /%compute gain coefficient b0
   b0=b(1) ; b= b/b0;
   a0--a(1); a=a/a0:
   b0 = b0/a0 ;
   /%
   M=length(b) ; N=length(a);
   if M＞N
   b=[b zeros(1,N-M)]; N=M;
   elseif M＞N
   a=[a zeros(1,M-N)]; N=M;
   else
   NM=0;
   end
   /%
   K=floor(N/2) ; B=zeros(K,3) ; A=zeros(K,3) ; A=zeros(K,3) ;
   if K*2=N;
   b=[b 0];
   a=[a 0];
   end
   /%
   broots = cplxpair( roots (b) ) ;
   broots= cplxpair(roots(a) ) ;
   for i=1: 2: 2*K
   Brow= broots( i: 1: i+1,: );
   Brow= real( poly(Brow));
   B(fix((i+1)/2),: )= Brow;
   Arow=aroots(i: 1: i+1,: );
   Arow = real ( poly(Arow) ) ;
   A(fix(i+1/2),: )= Arow;
   end
   MATLAB脚本
   ＞＞b=[1 -3 11 -27 18];
   ＞＞a=[l6 12 2 -4 -1 ];
   ＞＞[b0，B，A]=dircas(b，a)
   b0=0.0625
   B=
     1.0000  -0.0000  9.0000
     1.0000  -3.0000  2.0000
   A=
     1.0000   1.0000  0.5000
     1.0000  -0.2500  -0.1250