RP(n-1) model
The real-projective space (RP) model is given by
- Failed to parse (Conversion error. Server ("https://wikimedia.org/api/rest_") reported: "Cannot get mml. Server problem."): {\displaystyle H=-\sum _{x,\mu }(\sigma (x),\sigma (x,+\mu ))^{2}}
where H is the Hamiltonian of the system, the variables Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \sigma (x)} are n-component unit vectors of sites x of a square lattice, and Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \mu} denotes the two directions of the lattice. The n=2 case of the model is equivalent to the XY model.