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Karatsuba multiplication of two complex numbers can be performed with just three real multiplications (instead of four) as follows: $$(a+bi)(c+di) = (ac-bd) + i ((a+b)(c+d) - ac-bd)$$ We only need the products $ac$, $bd$ and $(a+b)(c+d)$. Does there exists a similar trick for quaternion multiplication? A naive multiplication would need $16$. This can probably be reduce to $9$ using the same trick as above (and taking into account that quaternion multiplication is non-commutative). My question is it known that $9$ is the lower bound? If not, does there exist an algorithm, which does quaternion multiplications in less than $9$ real multiplications?

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closed as off-topic by Will Jagy, darij grinberg, Joonas Ilmavirta, Alex Degtyarev, Dima Pasechnik Apr 24 '15 at 5:41

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