Spring 2018, Math 171
Week 5
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Reversibility/Detailed Balance Condition
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Show that Ehrenfest’s chain is reversible. \[P(x,y)= \begin{cases}\frac{N-x}{N}, & \mathrm{if\ } y=x+1 \cr \frac{x}{N}, & \mathrm{if\ } y=x-1 \cr 0, & \mathrm{otherwise}\end{cases}\]
- (Answer) Show KCC on simple cycles: All simple cycles are length 2, and cycles of length 2 always satisfy KCC.
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Consider the Markov chain \(\{X_n:n \ge 0\}\) on \(\mathcal{S} = \{1, 2, \dots, N\}\) with a transition matrix of the form \[P(x,x-1)=q(x), \; P(x,x)=r(x), \; P(x,x+1)=p(x)\]
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Find conditions on \(q, r, p\) which guarantee the chain will be irreducible.
- (Answer) \(q(x) \neq 0\) and \(p(x) \neq 0\) \(\forall x\)
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Show the chain is reversible
- (Answer) Show KCC on simple cycles: All simple cycles are length 2, and cycles of length 2 always satisfy KCC.
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Find the stationary distribution, assuming the chain is irreducible
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(Discussed) Consider the Markov chain \(\{X_n:n \ge 0\}\) on \(\mathcal{S} = \{1, 2, \dots, N\}\) with a transition matrix of the form \[P(x,y)= \begin{cases}q(x) & \mathrm{if\ } x \neq y \cr 1-(N-1)q(x) & \mathrm{if\ } x = y \end{cases}\] Verify that such chains are reversible
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Suppose \(P\) is the transition matrix for a reversible Markov chain. Fix \(i_0, j_0 \in \mathcal{S}\) Define a new transition matrix \[P'(x,y)= \begin{cases}aP(i_0, j_0) & \mathrm{if\ } x=i_0, y=j_0 \cr aP(j_0, i_0) & \mathrm{if\ } x=j_0, y=i_0 \cr P(i_0, i_0) + (1-a)P(i_0, j_0) & \mathrm{if\ } x=y=i_0 \cr P(j_0, j_0) + (1-a)P(j_0, i_0) & \mathrm{if\ } x=y=j_0 \cr P(x,y) & \mathrm{otherwise} \end{cases}\]
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Verify that the Markov chain defined by the new transition matrix will be reversible
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How will the stationary distribution of the new Markov chain relate to the stationary distribution of the original?
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Limiting Behavior
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(Discussed) Consider Ehrenfest’s chain \(\{X_n:n \ge 0\}\) subject to the transition probabilities. \[P(x,y)= \begin{cases}\frac{N-x}{N}, & \mathrm{if\ } y=x+1 \cr \frac{x}{N}, & \mathrm{if\ } y=x-1 \cr 0, & \mathrm{otherwise}\end{cases}\]
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Compute the period of \(\{X_n:n \ge 0\}\)
- (Answer) 2
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Show that \(Y_n=X_{2n}\) is a Markov chain. Is it irreducible?
- (Answer) No. The state space has been split into two parts which don’t communicate with each other.
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Consider \(Y_n=X_{2n}\) under the restriction \(X_0 \in \{0,\ 2,\ \dots,\ 2\lfloor\frac{N}{2}\rfloor\}\). Compute its stationary distribution. Explain why \(Y_n\) converges in distribution to the stationary distribution as \(n \to \infty\).
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(Discussed) Consider the Markov chain \(\{X_n:n \ge 0\}\) on \(\mathcal{S} = \{1, 2, \dots, N\}\) with a transition matrix of the form \[P(x,x-1)=q(x), \; P(x,x)=r(x), \; P(x,x+1)=p(x)\]
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Find conditions on \(q, r, p\) which lead to a period of 2
- (Answer) \(r(x) = 0\)
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Suppose \(q(x)=0\). Find a formula for \(\mathbb{E}_x[N(y)]\) for \(x < y\)
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Consider the Markov chain \(\{X_n:n \ge 0\}\) on \(\mathcal{S} = \{1, 2, \dots, N\}\) with a transition matrix of the form \[P(x,y)= \begin{cases}p, & \mathrm{if\ } y=x+1, \; x<N \cr 1-p, & \mathrm{if\ } x=0, \; y=0 \cr r & \mathrm{if\ } y = x, \; 0<y<N \cr q, & \mathrm{if\ } y=x-1, \; x>0 \cr 1-q, & \mathrm{if\ } x=N, \; y=N \cr 0, & \mathrm{otherwise}\end{cases}\]
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Find a system of equations which could be solved to find \(\mathbb{E}_x[T_N]\) for any \(x\).
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Compute \(\mathbb{E}_x[T_N]\) when \(r=0\) and \(q=1-p\)
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