function a = conjug(c)

% Discrete conjugation using complex FFT

n = length(c);
n1 = n/2;
a = fft(c);
a(1) = 0;
a(n1 + 1) = 0;
k = 2:n1;
a(k) =  - i*a(k);
a(n1 + k) =  i*a(n1 + k);
a = ifft(a);