Wednesday, April 24, 2024

A nice identity of the sum of a certain fraction series

I saw this question on stackexchange, and I was intrigued. The equation in question is,
k=1n22k1k(2n2knk)(2nn)=k=1n12k1
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
k=1n12k2n2n12n22n32n2k+22n2k+1=k=1n12k1
The first few n gives:
1221=1
1243+144321=1+13
1265+146543+16654321=1+13+15
1287+148765+16876543+1887654321=1+13+15+17
By grouping the terms, I came up with this expression:
87(12+65(14+43(16+21(18))))=1+13+15+17
An interesting thing that I noticed is, if I replace the numbers 12,14,16,18 with ones, it suddenly becomes obvious. We get this identity:
87+8765+876543+87654321=87(1+65(1+43(1+211)))=8=2+2+2+2
Then I wondered, what if I inverse the order of them? And I found that
1887+168765+14876543+1287654321=87(18+65(16+43(14+21(12))))=(1+11)(1+13)(1+15)(1+17)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, a1(b1+a2(b2+a3(b3+))), where ai and bj 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...

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