winding number can be smoothly deformed into each other and are said to belong to the same homotopy class. Gauge transformations which preserve the winding number are called small gauge transformations while ones that change the winding number are called large gauge transformations. For other non-abelian gauge groups it is sufficient to focus on one of their subgroups, ensuring that . This is because every mapping of onto can be continuously deformed into a mapping onto an subgroup of , a result that follows from Botts theorem. This is in contrast to abelian gauge groups where every mapping can be deformed to the constant map and so there is a single connected vacuum state. For a gauge field configuration , one can always calculate its winding number from a volume integral which in the temporal gauge is given by where is the coupling constant. The different classes of vacuum states with different winding numbers are referred to as topological vacua. Theta vacua Topological vacua are not candidate vacuum states of Yang–Mills theories since they are not eigenstates of large gauge transformations and so aren't gauge invariant. Instead acting on the state with a large gauge transformation with winding number will map it to a different topological vacuum . The true vacuum has to be an eigenstate of both small and large gauge transformations. Similarly to the form that eigenstates take in periodic potentials according to Bloch's theorem, the vacuum state is a coherent sum of topological vacua This set of states indexed by the angular variable are known as θ-vacua. They are eigenstates of both types of gauge transformations since now . In pure Yang–Mills, each value of will give a different ground state on which excited states are built, leading to different physics. In other words, the Hilbert space decomposes into superselection sectors since expectation values of gauge invariant operators between two different θ-vacua vanish if . Yang–Mills theories exhibit finite action solutions to their equations of motion called instantons. They are responsible for tunnelling between different topological vacua with an instanton with winding number being responsible for a tunnelling from a topological vacuum to . Instantons with are known as BPST instantons. Without any tunnelling the different θ-vacua would be degenerate, however instantons lift the degeneracy, making the various different θ-vacua physically distinct from each other. The ground state energy of the different vacua is split to take