What is the parameter 'k' in multiple feedback filter design?
In this tutorial from Analog devices, what is 'k'?

The 24th instance of k is: -
$$k = 2 π F_0 C5$$
On page 2. Also on page 2 is suggests k is: -
$$k = 2 π F_0 C1$$
But this is for a different circuit.
CTRL-F for &quot;find&quot; then enter &quot;k&quot;