# Period of each state

I am trying to determine the period of each state $$j = 0, 1, 2$$ for this irreducible Markov Chain with transition probability matrix $$P=\begin{bmatrix}0&0&1\\1&0&0\\\frac{1}{2}&\frac{1}{2}&0\end{bmatrix}$$

All states are of period $$1$$ because $$0 \rightarrow 2 \rightarrow 0$$ and $$0 \rightarrow 2 \rightarrow 1 \rightarrow 0$$ have $$gcd(2,3) =1$$

Is this correct and is the proof sound?

• Yes, the Markov Chain is aperiodic because of the two cycles you have identified whose lengths are relatively prime. – Fabio Somenzi Dec 8 '18 at 6:35

Comment: Here are some computations in R that I have sometimes found convenient for use with ergodic Markov Chains having small finite state spaces. [If the matrix $$\mathbf{P}$$ is based on empirical data, make sure each row of the transition matrix sums exactly to $$0.]$$

Establish ergodicity. $$\mathbf{P}^8$$ is a power of the transition matrix in the current Question that has all positive elements, so the chain is ergodic.

P = matrix(c(0, 0, 1,
1, 0, 0,
.5,.5, 0), nrow=3, byrow=T)
P
[,1] [,2] [,3]
[1,]  0.0  0.0    1
[2,]  1.0  0.0    0
[3,]  0.5  0.5    0

P2 = P %*% P;  P4 = P2 %*% P2;  P8 = P4 %*% P4;  P8
[,1]   [,2]   [,3]
[1,] 0.4375 0.1875 0.3750
[2,] 0.3750 0.2500 0.3750
[3,] 0.3750 0.1875 0.4375


Compute limiting distribution. The stationary vector $$\sigma$$ with $$\sigma\mathbf{P} = \sigma$$ is also the limiting distribution of the ergodic chain. For an ergodic matrix, the left eigenvector with the largest modulus is real and is proportional to the steady state vector.

eigen(t(P))$vectors # transpose to get LEFT eigenvectors [,1] [,2] [,3] [1,] -0.6666667+0i -0.3535534+0.3535534i -0.3535534-0.3535534i [2,] -0.3333333+0i -0.3535534-0.3535534i -0.3535534+0.3535534i [3,] -0.6666667+0i 0.7071068+0.0000000i 0.7071068+0.0000000i s = as.numeric(eigen(t(P))$vectors[,1]);  s = s/sum(s);  s
 0.4 0.2 0.4         # first-listed vector has largest modulus

s %*% P                 # to verify stationarity
[,1] [,2] [,3]
[1,]  0.4  0.2  0.4


So $$\sigma = (.4, .2, .4)$$ is the limiting distribution.

Perhaps there is more-elegant R code for this. If so, suggestions are welcome.