# paper a day : an LP inequality for concentration of measure

A Linear Programming Inequality with Applications to Concentration of Measure
Leonid Kontorovich
ArXiV: math.FA/0610712

The name “linear programming” is a bit of a misnomer –it’s not that Kontorovich comes up with a linear program whose solution is a bound, but more that the inequality relates two different norms, and the calculation of one of them can be thought of as maximizing an inner product over a convex polytope, which can be solved as a linear program. This norm inequality is the central fact in the paper. There are two weighted norms on functions at work — one is defined via a recursion that looks a lot like the sum of martingale differences. The other is maximizing the inner product of the given function with functions in a polytope defined by the weights. All functions in the polytope have bounded Lipschitz coefficient. By upper bounding the latter norm with the former, he can apply the former to the Azuma/Hoeffding/McDiarmid inequalities that show measure concentration for functions with bounded Lipschitz coefficient.

On a more basic level, consider a function of many random variables. For example, the empirical mean is such a function. A bounded Lipschitz coefficient means they are “insensitive” to fluctuations in one value. This intuitively means that as we add more variables the function will not deviate from is mean value by very much. To show this, you need to bound a probability with the Lipschitz coefficient, which is exactly what this inequality is doing. Kontorovitch thereby obtains a generalization of the standard inequalities.

What I’m not sure about is how I would ever use this result, but that’s my problem, really. The nice thing about the paper is that it’s short, sweet, and to the point without being terse. That’s always a plus when you’re trying to learn something.