Post History

*Note*: I can't comment on how this is generally approached in other subjects, but you're referencing Einstein, so as a physicist, I feel semi-qualified to weigh in on how we do it in physics. Note that this isn't necessarily general advice, but it likely applies elsewhere.

### Summary

"Einstein et al.'s ..." works OK, although in the majority of occassions I can think of, I'd rewrite to get rid of the possessive; "Einstein's et al. ..." is definitely not something I would use or have ever seen or heard of.

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### Details and examples

There are a few possible cases/options that apply here: While this list isn't necessarily comprehensive, you might want to...

1. use a **formal, 'standard' referencing convention**, in which case, every reference perhaps looks like something between square brackets e.g. `[1]` or `[Mit21]` or possibly an author (with et al.) and year e.g. `(Rincewind et al., 2021)` - no-one (that I know of/have seen) uses possessives of these, so it's not part of the 'standard convention', so the only way forward would be to reword the sentence.
> The gedankenexperiment ['thought experiment'] in [15] demonstrated ...

2. use an **informal standard referencing convention**. This is as per 1. but a bit more personal. I'd generally think of this in the first person, but in certain contexts, I see no reason it wouldn't work in the third person as well.
> In [5], we considered the use of ...

3. **mix the notation given in the question with standard citation**. I'd consider this to be the third person version of 2. although could also be made more formal.
> Cubitt et al. [12] proved ...

> For more details, see the proof in/by Cubitt et al. [12]

4. **stick with 'et al.' but remove possessiveness**. Could be useful if the paper and authors have already been mentioned. Would still recommend a proper citation.
> Squarepants et al. experimentally demonstrated that living underwater ...

5. **use the possessive form of  'et al.'**. I have only ever seen this as "et al.'s" but even this is relatively rare and I would generally recommend one of the other options above.
> Hong et al's experiment in the 1980s was...

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Generally, the closer to the top of the list, the more formal the expression but overall, the preferred expression depends a lot on the context and desired emphasis. If you're writing a presentation for a small group of people you already know, being more informal might actually be a good thing. If you're doing a presentation about the history of certain experiments, you're likely going to want a better way to refer to them than a number, so informally referring to them by author et al.'s names (i.e. just like that) would work quite well in this situation. However, if writing a paper intended for submission to a journal, it's likely that more formal expressions are more appropriate.