# Entropy of the sum of two vectors

Consider two identically distributed independent random vectors $X$ and $Y$ of dimension $n$ (assume $n$ is large). Both $X$ and $Y$ have only positive integer entries in some finite range. I am trying to understand what conditions are sufficient for $H(X) + H(Y) \approx H(X+Y)$?

Is it possible to say what properties of the random distributions are sufficient for this to be true?

You can say that $$H(X+Y)=H(X+Y | X) + H(X) - H(X | X+Y)=H(Y)+H(X)-H(X|X+Y),$$ so $H(X+Y) \approx H(X) + H(Y)$ iff $H(X|X+Y)\approx 0$, i.e., if you can essentially determine what $X$ must be when given $X+Y$. For instance, if the entries of $Y$ are always even, and the entries of $X$ can only be zero or one, then $H(X+Y)=H(X)+H(Y)$ exactly.
• This is very helpful but in my case $X$ and $Y$ are i.i.d. Is there a simple example that would work? – dorothy Dec 11 '15 at 21:36
• Entries of $X$ and $Y$ can be drawn from $\{1,2,4,8,\ldots, 2^n\}$. – mjqxxxx Dec 12 '15 at 1:12