Zero and pole of transfer function spaced for one octave
An octave is a factor of 2 in frequency.

If two frequencies \$f_1\$ and \$f_2\$ are spaced by an octave then either

$$\frac{f_1}{f_2} = 2$$

or 

$$\frac{f_1}{f_2} = \frac{1}{2}$$