I want to use the possessive noun form with et al. as in
et al.'s versus et al.
The grammatically correct way is "Einstein et al.'s paper", because to indicate possession by a noun phrase in English, the 's should be applied to the end of the phrase. However, I don't believe this specific construction is used much, as to me that looks somewhat unusual and feels informal. I'd recommend using a different wording like "the paper of Einstein et al." or "Einstein, Podolsky and Rosen's paper".
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