In the plot the values at the frequencies
\(2\pi j / N\), \(j=L+1-N,\ldots,L\), \(L:=\lfloor N/2 \rfloor\) are
shown, where \(N\) is the parameter specified on construction of the object
or \(N := 3\), if that parameter was smaller than three. A warning is given
in the later case.