function F = CSinterpFast(f)
y = fft(f);
n = length(f);
m = n/2;
a = (2/n)*real(y(1:m+1));
a(1) = a(1)/2;
a(m+1) = a(m+1)/2;
b = -(2/n)*imag(y(2:m));
F = struct('a',a,'b',b);