No input symbol should occur more frequently than 1-1/e
  Consider any discrete memoryless channel (DMC) with arbitrarily but finite input and output alphabets X, Y respectively. Then, for any capacity achieving input distribution all symbols occur less frequently than 1-1/e$. That is, \[ \max\limits_{x \in \mathcal{X}} P^*(x) < 1-\frac{1}{e} \] \noindent where $P^*(x)$ is a capacity achieving input distribution. Also, we provide sufficient conditions for which a discrete distribution can be a capacity achieving input distribution for some DMC channel. Lastly, we show that there is no similar restriction on the capacity achieving output distribution. 
Gowtham Kumar, Alexandros Manolakos
cs.IT math.IT