
> 
> $\textbf{Theorem}.1$ (The first Korn inequality) Suppose that $ \Omega $ is a bounded domain in $ \mathbb{R}^d $ with Lipschitz boundary. Then\
> \begin{eqnarray}
> \sqrt{2}\left\|\triangledown u\right\|\_{L^2(\Omega)}\leq \left\|\triangledown u+(\triangledown u)^T\right\|\_{L^2(\Omega)}
> \end{eqnarray}
> for any $ u\in H\_{0}^{1}(\Omega;\mathbb{R}^d) $, where $ (\triangledown u)^T $ denotes the transpose of $ \triangledown u $.
> 
> 
> 



> 
> $\textbf{Theorem}.2$ (The second Korn inequality) Suppose that $ \Omega $ is a bounded domain in $ \mathbb{R}^d $ with Lipschitz boundary. If $ u\in H^{1}(\Omega,\mathbb{R}^d) $ is a function with the property that $ u\perp R $ in $ H^{1}(\Omega;\mathbb{R}^d) $, then  
> 
> \begin{eqnarray}
> \int\_{\Omega}|\triangledown u|^2dx\leq C\int\_{\Omega}|\triangledown u+(\triangledown u)^T|^2dx
> \end{eqnarray}
> where $ R=\left\{\phi=Bx+b:B\in\mathbb{R}^{d\times d} \text{ is skew-symmetric and }b\in\mathbb{R}^d\right\} $ and $ C $ is a constant.
> 
> 
> 


I recently see the two theorems in a book about elliptic equations. I tried to get the estimate for the second inequality by direct computation which works in the proof of the first Korn inequality, but for this inequality, I cannot combine the condition $ u\perp R $ with the final results. Can you give me some hints or references?



How to prove the second Korn inequality?
You can find a full proof (to my knowledge the simpler one currently known) in the paper [1] and in the book [2], chapter I, §2.1 pp. 14-21. The original proof of Arthur Korn is so long and involved that K.O. Friedrichs, who gave a much simpler yet sophisticated proof, had doubts on his validity: starting from the work of Friedrichs, several authors gave their (in general quite complex) proofs, until Olga Oleĭnik gave a much shorter and simpler one (despite being still not elementary).


**New edit**. While ordering my library, I noted reference [1b]: in this paper Oleĭnik an Kondratiev prove the classical second Korn inequality for bounded domains satisfying the cone condition (theorem 1, a three page proof) and for certain classes of unbounded domains. They also prove that the constants in the inequality are sharp in some precise sense.


**References**


[1] Vladimir Alexandrovitch Kondratiev, Olga Arsenievna Oleĭnik,
"[On Korn’s inequalities](http://gallica.bnf.fr/ark:/12148/bpt6k6236782n/f497.image)" (English), Comptes Rendus de l’Académie des Sciences, Série I, 308, No. 16, pp. 483-487 (1989), [MR0995908](https://mathscinet.ams.org/mathscinet-getitem?mr=MR0995908), [Zbl 0698.35067](https://www.zbmath.org/?q=an%3A0698.35067).


[1b] Vladimir Alexandrovitch Kondrat’ev, Olga Arsenievna Oleĭnik, "Hardy’s and Korn’s type inequalities and their applications". (English) Rendiconti di Matematica e delle sue Applicazioni, VII Serie 10, No. 3, 641-666 (1990), [MR1080319](http://www.ams.org/mathscinet-getitem?mr=MR1080319), [Zbl 0767.35020](https://zbmath.org/0767.35020), also found in the commemorative book *Scritti matematici. Dedicati a Maria Adelaide Sneider*, Università "La Sapienza", 415-440 (1990).


[2] Olga Arsenievna Oleĭnik, Alexei Stanislavovich Shamaev, Grigorii Andronikovich Yosifian, *Mathematical problems in elasticity and homogenization*. (English) Studies in Mathematics and its Applications. 26. Amsterdam-London-New York-Tokyo: North- Holland, pp. xiii+398 (1992), ISBN: 0-444-88441-6, [MR1195131](https://mathscinet.ams.org/mathscinet-getitem?mr=MR1195131), [Zbl 0768.73003](https://www.zbmath.org/?q=an%3A0768.73003).


