# General covariant transformations

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In mathematics, **nonlinear realization** of a Lie group possessing a Cartan subgroup is a particular induced representation of . In fact it is a representation of a Lie algebra of in a neighborhood of its origin.

A nonlinear realization technique is part and parcel of many field theories with spontaneous symmetry breaking, e.g., nonlinear sigma model, chiral symmetry breaking, Goldstone boson theory, classical Higgs field theory, gauge gravitation theory and supergravity.

Let be a Lie group and its Cartan subgroup which admits a linear representation in a vector space . A Lie algebra of is split into the sum of the Cartan subalgebra of and its supplement so that

There exists an open neighbourhood of the unit of such that any element is uniquely brought into the form

Let be an open neighborhood of the unit of such that , and let be an open neighborhood of the -invariant center of the quotient which consists of elements

Then there is a local section of
over . With this local section, one can define the induced representation, called the **nonlinear realization**, of elements on given by the expressions

The corresponding nonlinear realization of a Lie algebra of takes the following form. Let , be the bases for and , respectively, together with the commutation relations

Then a desired nonlinear realization of in reads

up to the second order in . In physical models, the coefficients are treated as Goldstone fields. Similarly, nonlinear realization of Lie superalgebras is comsidered.

## See also

## References

- Coleman S., Wess J., Zumino B., Structure of phenomenological Lagrangians, I, II,
*Phys. Rev.***177**(1969) 2239. - Joseph A., Solomon A., Global and infinitesimal nonlinear chiral transformations,
*J. Math. Phys.***11**(1970) 748. - Giachetta G., Mangiarotti L., Sardanashvily G.,
*Advanced Classical Field Theory*, World Scientific, 2009, ISBN 978-981-283-895-7.

## External links

- Sardanashvily G., Reduction of principal superbundles, Higgs superfields, and supermetric. Appendix: Nonlinear realization, arXiv: hep-th/0609070.