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I got curious with the following: How can I find all the solutions for

$$\frac{1}{2^{k_1}} + \frac{1}{2^{k_2}} + \frac{1}{2^{k_3}} + \dots + \frac{1}{2^{k_n}}=1$$

for $k_i\in \Bbb{N}$ with $n$ a fixed positive integer? I thought about multiplying both sides by $2^{k_1} 2^{k_2}\dots 2^{k_n}$ but it looked useless at first sight. Is there some algorithm for that? Sorry if the question is too trivial, but I spent a while thinking and nothing came to my mind.

EDIT: I'm not sure this is actually number theory. Feel free to add or remove tags if it isn't.

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  • $\begingroup$ what are $k_1, k_2, \ldots$? $\endgroup$
    – gt6989b
    Mar 5, 2019 at 19:33
  • $\begingroup$ $k_i \in \Bbb{N}$ $\endgroup$
    – Red Banana
    Mar 5, 2019 at 19:46
  • 1
    $\begingroup$ One solution is $k_i=i$ for $1\le i\le (n-1)$ and $k_n=k_{n-1}$. $\endgroup$ Mar 5, 2019 at 19:49
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    $\begingroup$ You can generate all the solutions (up to a rearrangement) by constructing the full binary trees with $n$ leaves. Then, assign values of $k_i$ to be the level of the $i$th leaf on the tree. $\endgroup$ Mar 5, 2019 at 20:06
  • $\begingroup$ An interesting extension of the question is the replace the sum with a limit to infinity, where the k series would have to go through all the natural numbers. $\endgroup$ Mar 6, 2019 at 9:27

1 Answer 1

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This image shows how to generate iteratively all solutions $(k_1, \dots, k_n)$ for all $n$, under the constraint $k_1 \le \dots \le k_n$.

enter image description here

The "sons" of every solution $(k_1, \dots , k_i, k_{i+1} \dots , k_n)$ are found by replacing $k_i$ with $(k_i+1, k_i+1)$ (under the constraint that $k_i < k_{i+1}$).

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  • $\begingroup$ You missed $(1,2,4,4,4,4)$ for $n=6$ which is child of $(1,2,3,4,4)$. Also, $(1,3,3,3,4,4)$ is child of $(1,2,3,4,4)$ too. $\endgroup$ Mar 7, 2019 at 14:33
  • $\begingroup$ @mbjoe You are right! Thanks for your comment. I'm sorry, but I will not modify the image... $\endgroup$
    – Crostul
    Mar 7, 2019 at 18:40

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