# Show irreducibility of markov chain

I need to show that the markov chain that has transition matrix written below is irreducible. \begin{bmatrix} 0.2 & 0.5 & 0.1 & 0.1 & 0.1 \\ 0.2 & 0.5 & 0.3 & 0 & 0 \\ 0.2 & 0 & 0.4 & 0.4 & 0 \\ 0.2 & 0 & 0.2 & 0.4 & 0.2 \\ 0.2 & 0 & 0 & 0.1 & 0.7 \end{bmatrix}

Is it enough for me to say for $n = 1,2,3,4,5$ we have that $\mathbb{P}(X_1 = n | X_0 =1) > 0$ and $\mathbb{P}(X_1 = 1 | X_0 =n) > 0$? Hence its irreducible. Is there any other (easier) way to show irreducibility?

Also, to calculate the stationary distribution, is the fastest/most convenient way to use $\pi P = \pi$?

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You usually find the invariant measure using $\pi=\pi P$ and linear algebra. The invariant probability $\pi$ will be unique, since your chain is irreducible.

But your transition matrix is special, so there is a shortcut. The column sums of $P$ are all equal to one. Such a transition matrix is called doubly stochastic and its unique invariant probability measure is uniform, i.e., $\pi=\left({1\over 5},{1\over 5},{1\over 5},{1\over 5},{1\over 5}\right).$

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Thanks @ByronSchmuland ! Didn't realise that trick for doubly stochastic matrices, thanks for that. May I ask, is there any other way to show irreducibility apart from what I did above? –  Richard Apr 27 '12 at 21:16
@Richard Well, it amounts to the same thing, but you could calculate $P^2$ and note that all its entries are strictly positive. –  Byron Schmuland Apr 27 '12 at 21:23
Ahh okay, thanks. Just curious to see if there was any other way, but I think I'll stay with what I did. :) –  Richard Apr 27 '12 at 21:29