# Homework Help: Finding the rate of convergence for a markov chain

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1. Oct 25, 2015

### bonfire09

1. The problem statement, all variables and given/known data
For the following Markov chain, find the rate of convergence to the stationary distribution:
$\begin{bmatrix} 0.4 & 0.6 \\ 1 & 0 \end{bmatrix}$

2. Relevant equations

none

3. The attempt at a solution

I found the eigenvalues which were $\lambda_1=-.6$ or $\lambda_2=1$. The corresponding eigenvectors I found were $\vec{v_1}=\begin{bmatrix} -0.6 & 1 \end{bmatrix}$ and $\vec{v_2}=\begin{bmatrix} -1 & 1 \end{bmatrix}$. The stationary distribution which I found that satisfies $p=pA$(A is the transition matrix) and $p_1+p_2=1$ is $\vec{p}=\begin{bmatrix} .625 & .375 \end{bmatrix}$. From here I do not know how to get the rate of convergence. I think it has something to do with the eigenvalues or eigenvectors. Any help would be great thanks.

Last edited: Oct 25, 2015
2. Oct 25, 2015

### andrewkirk

3. Oct 25, 2015

### bonfire09

I checked the eigenvectors. I had them switched. So do I use the formula right below where it "since the eigenvectors are orthonormal..."?

4. Oct 26, 2015

### andrewkirk

That depends on what you think it does. It's best to only use formulas that one understands. Without going through every step of the maths, do you understand broadly why that formula has that form?

5. Oct 26, 2015

### bonfire09

Actually I don't. We talked about limiting distributions and stationary distributions in class but did not cover rate of convergence. In essence I only understand the very basics. So somehow I have to show the rate of convergence with what I have so far.

6. Oct 26, 2015

### andrewkirk

Well, the matrix eigenvectors will be orthogonal and hence form a basis for the vector space. So any vector $\mathbf{x}$ can be expressed as a linear sum $\sum_{i=1}^na_i\mathbf{u}_i$ of eigenvectors, as shown in the wiki article. If only one of the eigenvalues, say $\lambda_1$, corresponding to eigenvector $\mathbf{u}_1$, has absolute value equal to 1, and all others have absolute values less than 1, what will happen to $\mathbf{x}=\sum_{i=1}^na_i\mathbf{u}_i$ as you repeatedly right-multiply it by the matrix? Think about the impact of each term in the sum separately.

7. Oct 26, 2015

### Ray Vickson

Please tun off the bold font, reserving it for emphasis on individual words, etc, like what I do below.

Anyway, do you know the definition of rate of convergence? Is this discussed in your textbook or course notes? Even if it is in a section of your text that you have not reached yet, I would bet you would be allowed to read it ahead of time and use the material as needed. If such material is not in your text or course notes, please tell us so we can point you in the appropriate direction.