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Pokhozhaev's identity is an integral relation satisfied by stationary localized solutions to a nonlinear Schrödinger equation or nonlinear Klein–Gordon equation. It was obtained by S.I. Pokhozhaev and is similar to the virial theorem. This relation is also known as G.H. Derrick's theorem. Similar identities can be derived for other equations of mathematical physics.

The Pokhozhaev identity for the stationary nonlinear Schrödinger equation

Here is a general form due to H. Berestycki and P.-L. Lions.

Let ${\displaystyle g(s)}$ be continuous and real-valued, with ${\displaystyle g(0)=0}$. Denote ${\displaystyle G(s)=\int _{0}^{s}g(t)\,dt}$. Let

${\displaystyle u\in L_{\mathrm {loc} }^{\infty }(\mathbb {R} ^{n}),\qquad \nabla u\in L^{2}(\mathbb {R} ^{n}),\qquad G(u)\in L^{1}(\mathbb {R} ^{n}),\qquad n\in \mathbb {N} ,}$

be a solution to the equation

${\displaystyle -\nabla ^{2}u=g(u)}$,

in the sense of distributions. Then ${\displaystyle u}$ satisfies the relation

${\displaystyle {\frac {n-2}{2}}\int _{\mathbb {R} ^{n}}|\nabla u(x)|^{2}\,dx=n\int _{\mathbb {R} ^{n}}G(u(x))\,dx.}$

The Pokhozhaev identity for the stationary nonlinear Dirac equation

There is a form of the virial identity for the stationary nonlinear Dirac equation in three spatial dimensions (and also the Maxwell-Dirac equations) and in arbitrary spatial dimension. Let ${\displaystyle n\in \mathbb {N} ,\,N\in \mathbb {N} }$ and let ${\displaystyle \alpha ^{i},\,1\leq i\leq n}$ and ${\displaystyle \beta }$ be the self-adjoint Dirac matrices of size ${\displaystyle N\times N}$:

${\displaystyle \alpha ^{i}\alpha ^{j}+\alpha ^{j}\alpha ^{i}=2\delta _{ij}I_{N},\quad \beta ^{2}=I_{N},\quad \alpha ^{i}\beta +\beta \alpha ^{i}=0,\quad 1\leq i,j\leq n.}$

Let ${\displaystyle D_{0}=-\mathrm {i} \alpha \cdot \nabla =-\mathrm {i} \sum _{i=1}^{n}\alpha ^{i}{\frac {\partial }{\partial x^{i}}}}$ be the massless Dirac operator. Let ${\displaystyle g(s)}$ be continuous and real-valued, with ${\displaystyle g(0)=0}$. Denote ${\displaystyle G(s)=\int _{0}^{s}g(t)\,dt}$. Let ${\displaystyle \phi \in L_{\mathrm {loc} }^{\infty }(\mathbb {R} ^{n},\mathbb {C} ^{N})}$ be a spinor-valued solution that satisfies the stationary form of the nonlinear Dirac equation,

${\displaystyle \omega \phi =D_{0}\phi +g(\phi ^{\ast }\beta \phi )\beta \phi ,}$

in the sense of distributions, with some ${\displaystyle \omega \in \mathbb {R} }$. Assume that

${\displaystyle \phi \in H^{1}(\mathbb {R} ^{n},\mathbb {C} ^{N}),\qquad G(\phi ^{\ast }\beta \phi )\in L^{1}(\mathbb {R} ^{n}).}$

Then ${\displaystyle \phi }$ satisfies the relation

${\displaystyle \omega \int _{\mathbb {R} ^{n}}\phi (x)^{\ast }\phi (x)\,dx={\frac {n-1}{n}}\int _{\mathbb {R} ^{n}}\phi (x)^{\ast }D_{0}\phi (x)\,dx+\int _{\mathbb {R} ^{n}}G(\phi (x)^{\ast }\beta \phi (x))\,dx.}$

See also

• Virial theorem
• Derrick's theorem

References

1. Pokhozhaev, S.I. (1965). "On the eigenfunctions of the equation ${\displaystyle \Delta u+\lambda f(u)=0}$". Dokl. Akad. Nauk SSSR. 165: 36–39.
2. Berestycki, H. and Lions, P.-L. (1983). "Nonlinear scalar field equations, I. Existence of a ground state". Arch. Rational Mech. Anal. 82 (4): 313–345. Bibcode:1983ArRMA..82..313B. doi:10.1007/BF00250555. S2CID 123081616.{{cite journal}}: CS1 maint: multiple names: authors list (link)
3. Esteban, M. and Séré, E. (1995). "Stationary states of the nonlinear Dirac equation: A variational approach". Commun. Math. Phys. 171 (2): 323–350. Bibcode:1995CMaPh.171..323E. doi:10.1007/BF02099273. S2CID 120901245.{{cite journal}}: CS1 maint: multiple names: authors list (link)
4. Boussaid, N. and Comech, A. (2019). Nonlinear Dirac equation. Spectral stability of solitary waves. Mathematical Surveys and Monographs. Vol. 244. American Mathematical Society. doi:10.1090/surv/244. ISBN 978-1-4704-4395-5. S2CID 216380644.{{cite book}}: CS1 maint: multiple names: authors list (link)

• Mathematical identities
• Theorems in mathematical physics
• Physics theorems