RP(n-1) model
The real-projective space (RP) model is given by
- 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 \left. H \right. = - \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.