y=@(x)c1*exp(-diff(x))+d*(sin(x)/(d^2+1))+d^2*(cos(x)/d^2+1);
numel(diff(x)) is one less than numel(x) so the above will fail owing to mismatch in number of elements.
But, since
x=-pi:0.01:pi;
it is so that
diff(x) --> 0.01 % within rounding error
so may as well just write as
dx=0.01;
x=-pi:dx:pi;
exp_dx=exp(-dx);
y=@(x) c1*exp_dx+d*(sin(x)/(d^2+1))+d^2*(cos(x)/d^2+1);
