x = [0:0.1:0.3];
  n=3;
  f = cos(x);
t=0;
alg431

  p = diag(a);
xx = [0:0.001:0.4];


y1 = nip(p,1,x,xx)-cos(xx);
y2 = nip(p,2,x,xx)-cos(xx);
y3 = nip(p,3,x,xx)-cos(xx);
figure(2), semilogy(xx,abs(y1),xx,abs(y2),'r',xx,abs(y3),'k')
  figure(1),plot(xx,y1,xx,y2,'r',xx,y3,'k')