I'm looking at Eqn. 12 here
I tried the power rule $a^m \div a^n = a^{m-n}$ to get $2^1 / 2^s$. But probably this is wrong.
Looking at the equation this should probably be $1/2^s$.
Can you help?
EDIT:
I see in this question that the same expression is written in reverse as $2^{s-1}$ so this may be literally 2 raised to the power of $(1-s)$.