Solving binomial coefficient inequality

185 Views Asked by At

I have the following inequality, $$\binom{\binom{n}{k}}{a}\binom{\binom{n}{k-l}}{b}<\binom{\binom{n}{k}}{a+b}\tag 1$$ I was wondering how one solves this exact / approximately for l?

1

There are 1 best solutions below

0
On

If you substitute all combination expressions inside and open all the brackets, you will end up with a fraction $\frac{P(l)}{Q(l)} < 0$, where $P(l)$ and $Q(l)$ are polynomials. You can then attempt to factorize each polynomial into linear multipliers and apply the standard high-school polynomial inequality solution method to determine for which intervals of the real line is the LHS of the equation positive and for which negative, and then determine which integer values of $l$ fit into the positive intervals.

However, if you do that, you will get into a giant mess, because factoring a polynomial containing products of products of factorials will give you thousands of zeros on your real line. Since $l$ is integer, why not just write a computer program checking if the inequality holds for each value of $l$ from $0$ to $k$?