Sigma model

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In physics, a sigma model is a physical system which is described by a Lagrangian density of the form

\mathcal{L}(\phi_1, \phi_2, \ldots, \phi_n) = \sum_{i=1}^n \sum_{j=1}^n g_{ij} \; \mathrm{d}\phi_i \wedge {*\mathrm{d}\phi_j}

Depending on the scalars gij it is either a linear sigma model or a non-linear sigma model.

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