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I have a variable $K$, that is the sum of its components $K_a$, $K_b$, $K_c$. Now I want to describe a set of $K$'s, but I can't use $K_i$ because I already used the subscript notation to differentiate between the components of $K$.

I don't want to call the components e.g. $a$, $b$, and $c$, because it would be confusing, since they're less conceptually connected, and I'd also be running out of alphabet.

I also don't want to mix this with function notation, e.g. $K(i)$ or $K_a(i)$ because that's also confusing, since this is a set.

I also don't want to double subscript, e.g. $K_{a_i}$ since that's even more confusing and conflates components with values in the set.

Are there any other alternatives?

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    $\begingroup$ Is something like $K_{ij}$ or $K_{i,j}$ viable? On the other hand, if you make it clear what you mean, $K^i_a$ is fine as well, and the same goes for $K(i)$. $\endgroup$ – Shuri2060 Jul 24 '17 at 10:15
  • $\begingroup$ I think I would prefer the superscript, because I don't want people to think that $a$ is an index. If you think that makes the most sense, please do feel free to put it as an answer so I can accept it. $\endgroup$ – Yousef Amar Jul 24 '17 at 10:19
  • $\begingroup$ if you had them numbered instead of lettered for the index you could call them $K_j$ and sum them with $\sum_j K_j$ etc. $\endgroup$ – user451844 Jul 24 '17 at 10:20
  • $\begingroup$ @RoddyMacPhee The problem is that each denotes a specific thing that I want to refer to (e.g. $K_a$ is some fixed baseline). I'm not sure how that kind of thing is usually written. $\endgroup$ – Yousef Amar Jul 24 '17 at 10:22
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    $\begingroup$ The same problem is often encountered with vectors and components. Although it might be a pain to write the brackets, I denote the second component of the first standard basis vector as ${(e_1)}_2$ $\endgroup$ – Shuri2060 Jul 24 '17 at 10:27
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Superscripts are commonly used as indexes. If you want to make sure it is not confused with raising to a power you can put them in parentheses like $K_a^{(x)}$ or something like that.

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$$\mathbf{K}=\{\mathbf{K}_i: i \in I\},~\mathbf{K}_i=K_{ai}+K_{bi}+K_{ci}$$

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