My question is about this article:

Is there a calculation error here:

I used Euler’s formula for T_{s} - T_{-s} elements and got this:

D = \frac{\gamma}{sin{(2\gamma s)}} (T_{s} - T_{-s})

And the same is for the derivative:

f'(\theta) = \frac{\gamma}{sin{(2\gamma s)}}[f(\theta + s) - f(\theta - s)]

Am I wrong or not?