I know questions about this warning have been posted a thousand times over. But I really couldn't find any solution that worked in my case. It's quite simple:
f[a_] := (1 - 1/a)*-(1/((a - 1)*(c - 1)))*Log[2, (1/((a - 1)*(c - 1)))] - (1 - (1/((a - 1)*(c - 1))))*Log[2, 1 - (1/((a - 1)*(c - 1)))]; Solve[f'[a] == 0, a]
I know from plotting the function that there is a unique solution somewhere around $a = c/10$, but it seems like Mathematica does not find it.
If anybody could help, I would be supremely grateful.