Nonlinear realization

In mathematical physics, nonlinear realization of a Lie group G possessing a Cartan subgroup H is a particular induced representation of G. In fact, it is a representation of a Lie algebra g {displaystyle {mathfrak {g}}} of G in a neighborhood of its origin.A nonlinear realization, when restricted to the subgroup H reduces to a linear representation. In mathematical physics, nonlinear realization of a Lie group G possessing a Cartan subgroup H is a particular induced representation of G. In fact, it is a representation of a Lie algebra g {displaystyle {mathfrak {g}}} of G in a neighborhood of its origin.A nonlinear realization, when restricted to the subgroup H reduces to a linear representation. A nonlinear realization technique is part and parcel of many field theories with spontaneous symmetry breaking, e.g., chiral models, chiral symmetry breaking, Goldstone boson theory, classical Higgs field theory, gauge gravitation theory and supergravity. Let G be a Lie group and H its Cartan subgroup which admits a linear representation in a vector space V. A Liealgebra g {displaystyle {mathfrak {g}}} of G splits into the sum g = h ⊕ f {displaystyle {mathfrak {g}}={mathfrak {h}}oplus {mathfrak {f}}} of the Cartan subalgebra h {displaystyle {mathfrak {h}}} of H and its supplement f {displaystyle {mathfrak {f}}} , such that (In physics, for instance, h {displaystyle {mathfrak {h}}} amount to vector generators and f {displaystyle {mathfrak {f}}} to axial ones.) There exists an open neighborhood U of the unit of G suchthat any element g ∈ U {displaystyle gin U} is uniquely brought into the form Let U G {displaystyle U_{G}} be an open neighborhood of the unit of G such that U G 2 ⊂ U {displaystyle U_{G}^{2}subset U} , and let U 0 {displaystyle U_{0}} be an open neighborhood of theH-invariant center σ 0 {displaystyle sigma _{0}} of the quotient G/H which consists of elements Then there is a local section s ( g σ 0 ) = exp ⁡ ( F ) {displaystyle s(gsigma _{0})=exp(F)} of G → G / H {displaystyle G o G/H} over U 0 {displaystyle U_{0}} . With this local section, one can define the induced representation, called the nonlinear realization, of elements g ∈ U G ⊂ G {displaystyle gin U_{G}subset G} on U 0 × V {displaystyle U_{0} imes V} given by the expressions The corresponding nonlinear realization of a Lie algebra g {displaystyle {mathfrak {g}}} of G takes the following form.