I want to use the possessive noun form with et al. as in
et al.'s versus et al.
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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".