N=100; a=-5; b=5;
hx=(b-a)/N;      % PASSO INTP. PTS.
x=a:hx:b;        % ASCISSE (INTP).
y=runge(x);      % ORDINATE (INTP).
h1=hx/4;         % PASSO TEST PTS.
u=a:h1:b;        % ASCISSE (TEST).
v = interp1(x,y,u,'linear');  % ORDINATE (TEST).
vv=runge(u);     % VALORE ESATTO IN NODI TEST.
err=norm(vv-v,inf);   % ERRORE ASSOLUTO IN  
                      % NORMA INFINITO.
fprintf('\n \t [ERR]: %2.2e',err);
plot(u,v,'k-',u,vv,'r-');  % PLOT SPLINE VS. RUNGE.
fprintf('\n \t [PAUSE]');
pause(5);
semilogy(u,abs(vv-v),'+');  % PLOT ERRORE.