# Inequality for Stirling numbers of the second kind

I stumbled upon the following inequality which, I believe, is true. I was able to prove it for small k, but I have no proof for the general case. Any help is welcome.

Let $n\geq k\geq 1$ then $$\left(1+\frac{1}{n}\right)^k S(n+1,k+1)\geq \left(1+\frac{1}{k}\right)^n S(n,k)$$ where $S(n,k)$ is a Stirling number of second kind.

• Temme (Studies in applied mathematics 89, 23-243) gave an asymptotic expression for Stirling numbers. Did you check whether it suffices to yield your inequality, at least for a certain range of $n,k$? – Jan-Christoph Schlage-Puchta Feb 12 '17 at 14:48
• this should be very neat, say, for $n=k+2$ many terms of Taylor expansion coincide – Fedor Petrov Feb 13 '17 at 10:47

Let me prove a weaker inequality, the proof of which allows further improvements. Note that $S(n,k)=h_{n-k}(1,2,\dots,k)$, where $$h_t(x_1,\dots,x_k)=\sum_{a_1+\dots+a_k=t,a_i\geqslant 0} x_1^{a_1}\dots x_k^{a_k}$$ is a complete symmetric polynomial. Thus $$\frac{S(n+1,k+1)}{S(n,k)}=\frac{h_{n-k}(1,2,\dots,k+1)}{h_{n-k}(1,2,\dots,k)}=\frac{\sum(p_1+1)\dots (p_{n-k}+1)}{\sum p_1\dots p_{n-k}},$$ where summation is taken over $0\leqslant p_1\leqslant p_2\leqslant \dots \leqslant p_{n-k}\leqslant k$. For any such $(n-k)$-tuple we have $$(p_1+1)\dots (p_{n-k}+1)\geqslant \left(1+\frac1k\right)^{n-k}p_1\dots p_{n-k},$$ summing up we get $$S(n+1,k+1)\geqslant \left(1+\frac1k\right)^{n-k} S(n,k),$$ this is worse than you need, but the inequality was not sharp enough. Roughly speaking, you need to prove that "in average" the ratio $(1+1/p_1)\dots (1+1/p_{n-k})$ grows as $(1+2/k)^{n-k}$, that corresponds to "average" $p_i\sim k/2$.