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: Looking at it more closely again it does not seem a remarkable or interesting result. It's wrong to say it derives the identity without knowing it; it assumes the form of the identity with quaternion unknowns then does some very longwinded calculations to work out the unknowns. A pointless exercise in algebra. There are far better (shorter, clearer and more interesting) geometric derivations using [[geometric algebra]]. Not everything needs a derivation but that could perhaps be added. But your derivation or a link to it I don't think helps at all.--<small>[[User:JohnBlackburne|JohnBlackburne]]</small><sup>[[User_talk:JohnBlackburne|words]]</sup><sub style="margin-left:-2.0ex;">[[Special:Contributions/JohnBlackburne|deeds]]</sub> 03:14, 6 January 2015 (UTC)
:: It doesn't assume the form of the identity. It starts by noting the striking similarities between the terms present in quaternion multiplication and the terms present in the rotation function. It's conceivable that a person could have seen these similarities even if they hadn't known in advance about how unit quaternions do rotations. The processes is a plausible path of first discovery, and it's therefore a "derivation." It's only "long winded" because quaternion multiplication involves so many terms; you might also call general relativity "long winded" in the same sense. I resent that you don't trust my intentions, and I also resent that you just admitted to not having even looked at the material before doing your revert.
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