I saw this question on stackexchange, and I was intrigued. The equation in question is, $$\sum_{k=1}^n\frac{2^{2k-1}}{k}\frac{\binom{2n-2k}{n-k}}{\binom{2n}{n}}=\sum_{k=1}^n\frac{1}{2k-1}$$ Are you able to prove this identity without looking at the proof that I gave under the question? After simplifying the expression, it turned into $$\sum_{k=1}^n\frac{1}{2k}\frac{2n}{2n-1}\frac{2n-2}{2n-3}\dots\frac{2n-2k+2}{2n-2k+1}=\sum_{k=1}^n\frac{1}{2k-1}$$ The first few $n$ gives: $$\frac{1}{2}\frac{2}{1}=1$$ $$\frac{1}{2}\frac{4}{3}+\frac{1}{4}\frac{4}{3}\frac{2}{1}=1+\frac{1}{3}$$ $$\frac{1}{2}\frac{6}{5}+\frac{1}{4}\frac{6}{5}\frac{4}{3}+\frac{1}{6}\frac{6}{5}\frac{4}{3}\frac{2}{1}=1+\frac{1}{3}+\frac{1}{5}$$ $$\frac{1}{2}\frac{8}{7}+\frac{1}{4}\frac{8}{7}\frac{6}{5}+\frac{1}{6}\frac{8}{7}\frac{6}{5}\frac{4}{3}+\frac{1}{8}\frac{8}{7}\frac{6}{5}\frac{4}{3}\frac{2}{1}=1+\frac{1}{3}+\frac{1}{5}+\frac{1}{7}$$ By grouping the terms, I came up with this expression: $$\frac{8}{7}\left(\frac{1}{2}+\frac{6}{5}\left(\frac{1}{4}+\frac{4}{3}\left(\frac{1}{6}+\frac{2}{1}\left(\frac{1}{8}\right)\right)\right)\right)=1+\frac{1}{3}+\frac{1}{5}+\frac{1}{7}$$ An interesting thing that I noticed is, if I replace the numbers $\frac{1}{2},\frac{1}{4},\frac{1}{6},\frac{1}{8}$ with ones, it suddenly becomes obvious. We get this identity: $$\frac{8}{7}+\frac{8}{7}\frac{6}{5}+\frac{8}{7}\frac{6}{5}\frac{4}{3}+\frac{8}{7}\frac{6}{5}\frac{4}{3}\frac{2}{1}=\frac{8}{7}\left(1+\frac{6}{5}\left(1+\frac{4}{3}\left(1+\frac{2}{1}1\right)\right)\right)=8=2+2+2+2$$ Then I wondered, what if I inverse the order of them? And I found that $$\frac{1}{8}\frac{8}{7}+\frac{1}{6}\frac{8}{7}\frac{6}{5}+\frac{1}{4}\frac{8}{7}\frac{6}{5}\frac{4}{3}+\frac{1}{2}\frac{8}{7}\frac{6}{5}\frac{4}{3}\frac{2}{1}=\frac{8}{7}\left(\frac{1}{8}+\frac{6}{5}\left(\frac{1}{6}+\frac{4}{3}\left(\frac{1}{4}+\frac{2}{1}\left(\frac{1}{2}\right)\right)\right)\right)=\left(1+\frac{1}{1}\right)\left(1+\frac{1}{3}\right)\left(1+\frac{1}{5}\right)\left(1+\frac{1}{7}\right)-1$$ which is very easy to prove. The proof of the first equation, though, is much harder to construct, because the coefficient changes in the opposite direction of $n$ from the inside to the outside, so each time $n$ changes, the previous expression is no longer a part of the new expression, making the induction argument difficult to find. But after some manipulation, I found the proof, which I posted on the stackexchange page, so I won't repeat it here. I wonder if more simple sequences can be constructed with other sequences as the coefficients This reminds me of continued fraction. I think maybe this should have a name, like, continued product? But it seems that this name has already been taken by something different. So, maybe "rolling product"? A similar concept seems to be Engel expansion, but here we have a way more general form, $a_1(b_1+a_2(b_2+a_3(b_3+\dots)))$, where $a_i$ and $b_j$ are rational, while the result is the sum of a different series. Or maybe this can further be generalized to any real sequences, or even complex sequences? I wonder...
Wednesday, April 24, 2024
A nice identity of the sum of a certain fraction series
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