I saw this question on stackexchange, and I was intrigued. The equation in question is,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 The first few gives: By grouping the terms, I came up with this expression: An interesting thing that I noticed is, if I replace the numbers with ones, it suddenly becomes obvious. We get this identity: Then I wondered, what if I inverse the order of them? And I found that 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 from the inside to the outside, so each time 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, , where and 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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