Ehrenfest urn stationary distribution

In an Ehrenfest model with NN balls split between two urns, a uniformly random ball switches urns each step. What is the stationary distribution of the number of balls in urn 1?

Show hints (2)+
  1. Check detailed balance against a binomial guess.
  2. Each ball is independently in urn 1 with probability 1/21/2 at stationarity.

Answer

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Binomial(N,1/2)\text{Binomial}(N, 1/2)

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