Odd-order Butterworth filter gain at cutoff frequency
Congratulations, you've found an error on the wikipedia page on Butterworth filters. The squared magnitude of the frequency response of an $n^{th}$-order Butterworth low pass filter is

$$|H(j\omega)|^2=\frac{1}{1+\left(\frac{\omega}{\omega_c}\right)^{2n}}\tag{1}$$

There should be no imaginary unit $j$ in that formula.