% FIGURE 5.3.3b
      function USOL = USOL(IDER,X,I1,I2)
      global M
      global A
      global OMEGA
%                              EVALUATE THE APPROXIMATE SOLUTION (IDER=0)
%                              OR ITS IDER-TH DERIVATIVE AT X
      USOL = feval(OMEGA,IDER,X);
%                              PHI(I) IS NONZERO AT X ONLY FOR I=I1,I2
      for I=max(I1,1):min(I2,M)
         USOL = USOL + A(I)*PHI(I,IDER,X);
      end