Here is another identity but this time corresponding to an eight-fold: the Eight-square identity of Ferdinand Degen found about 1818. You know the drill: it states that a product of two numbers that are each the sum of eight squares is itself the sum of eight squares!

(a_{1}^{2} + a_{2}^{2} + a_{3}^{2} + a_{4}^{2} + a_{5}^{2} + a_{6}^{2} + a_{7}^{2} + a_{8}^{2})(b_{1}^{2} + b_{2}^{2} + b_{3}^{2} + b_{4}^{2} + b_{5}^{2} + b_{6}^{2} + b_{7}^{2} + b_{8}^{2}) =

…The sum of the expressions in the eight triangles written in the diagram above. (Please consult the Wikipedia entry below for the textual formulas, as it’s too hard to write in HTML.)

Note that the expressions above have an interesting symmetry, aside from the one on the upper left. Indeed, Euler’s Four-square Identity has a similar simpler symmetry. There is also a connection with Octonions if you are interested in digging for it. If you are anticipating that there is such a formula for sums of sixteen squares, there is, but not a bilinear one, and it is much more complicated!

Further Reading:

https://en.wikipedia.org/wiki/Degen%27s_eight-square_identity

https://en.wikipedia.org/wiki/Pfister%27s_sixteen-square_identity

[*11.64]

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