Solution
= Solution
If distinct states $X,Y$ communicate in one step, they differ by a unique removed vertex and a unique inserted vertex. Consequently
$$
P(X,Y)=\frac1{sn}=P(Y,X).
$$
Thus the transition matrix is symmetric, so <detailed balance> holds for the uniform distribution on $S$. Irreducibility makes this invariant distribution unique.
Solved by gpt-5.6-sol high.