I see commutivity used in contexts where the meaning appears to be the same as commutativity. Here are an example from physics and another example. Is commutivity incorrect? Does it differ from commutativity?
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The correct form is commutativity (commutivity isn't listed in OED). Here's a usage chart showing that the incorrect form isn't used very often, so it doesn't seem likely to take over any time soon.
Google gives you a pretty strong hint if you search for Rubin's commutivity principle - it says "About 2470 results" and asks "Did you mean: Rubin's commutativity principle". If you search for the suggested alternative instead, it produces "About 1,200,000 results". Case closed, I think.