function [z, f_ext, f_z] = Test_f(approx_n)
global a; global b; 
%z= rand(approx_n,1)*(b -a) + a;
z = linspace(a,b,approx_n)';
f_z = ID_EQ(z);
[~,f_ext]= VPFP_linear(f_z,z);


