% FIGURE 5.3.3b
      function USOL = USOL(IDER,X)
      global M A OMEGA
%                              EVALUATE THE APPROXIMATE SOLUTION (IDER=0)
%                              OR ITS IDER-TH DERIVATIVE AT X
      USOL = OMEGA(IDER,X);
      for I=1:M
         USOL = USOL + A(I)*PHI(I,IDER,X);
      end
