A note on $\sigma$-model with the target $S^n$

Naively the Hilbert space of a sigma model has to be defined as an L^2 space of functions on the space of free loops of the target. This object is not well defined. In this note we study a finite-dimensional approximations L_N(S^n) of the free loops of the sphere S^n. Spaces L_N(S^n) are defined in terms of finite Fourier series. L_N(S^n) finite-dimensional but singular. We compute Riemann and Ricci curvatures of the smooth locus of this space and study Schr"odinger operator in the case of L_1(S^n)