Hua's identity

In algebra, Hua's identity^[1] states that for any elements a, b in a division ring,

a-(a^{{-1}}+(b^{{-1}}-a)^{{-1}})^{{-1}}=aba

whenever $ab\neq 0,1$ . Replacing $b$ with $-b^{{-1}}$ gives another equivalent form of the identity:

(a+ab^{{-1}}a)^{{-1}}+(a+b)^{{-1}}=a^{{-1}}.

An important application of the identity is a proof of Hua's theorem.^[2]^[3] The theorem says that if $\sigma :K\to L$ is a function between division rings and if $\sigma$ satisfies:

\sigma (a+b)=\sigma (a)+\sigma (b),\quad \sigma (1)=1,\quad \sigma (a^{{-1}})=\sigma (a)^{{-1}},

then $\sigma$ is either a homomorphism or an antihomomorphism. The theorem is important because of the connection to the fundamental theorem of projective geometry.

Proof

(a-aba)(a^{-1}+(b^{-1}-a)^{-1})=1-ab+ab(b^{-1}-a)(b^{-1}-a)^{-1}=1.

References

↑ Cohn 2003, §9.1
↑ Cohn 2003, Theorem 9.1.3
↑ "Is this map of domains a Jordan homomorphism?". math.stackexchange.com. Retrieved 2016-06-28.

Cohn, Paul M. (2003). Further algebra and applications (Revised ed. of Algebra, 2nd ed.). London: Springer-Verlag. ISBN 1-85233-667-6. Zbl 1006.00001.

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[1] Cohn 2003, §9.1

[2] Cohn 2003, Theorem 9.1.3

[3] "Is this map of domains a Jordan homomorphism?". math.stackexchange.com. Retrieved 2016-06-28.