That may make sense internal to a library, where you’ve established idioms.

But as a counterpoint, I only know what you meant by the first expression because I read the second expression.

I actually agree with you in large part, that short variable names can have more meaning within an established set of idioms because they allow you to parse whole statements at once. But there’s a trade-off involved, because mathematics can take symbology further than that’s useful.

For example:

    E[i=0;5](i**2)

    sum([i**2 for i in range(0,5)])

So it often comes down to a matter of taste.

Why not something like this?

    position = C * state + D * input

Reasoning: k is the only subscript, so it can be dropped. Meaning of C and D are implicitly defined through their function wrt to state and input, so its OK not to name them. It also keeps the structure visible similar to that of the math.