2 problems

The $O(3)$ sigma model has topological charge … where … The Euclidean action may include the theta term ${\rm i}\vartheta {\cal Q}$. Haldane's conjecture. When $\vartheta=\pi$, the…

The $O(3)$ sigma model, the $SU(N)/SO(N)$ sigma models, and the $O(2P)/O(P) \times O(P)$ models have a ${\mathbb Z}_2$ instanton number, so their theta angle takes the values…