# A basic question on necessary and sufficient condition for positive recurrence

If state $j$ is recurrent and the following holds can it be called as positive recurrent ?

$$\lim_{n -> \infty}\frac{1}{n}\sum_{k=1}^{n}p_{jj}^{(k)} > 0$$

I know that this a necessary condition for positive recurrence, but is it sufficient ?

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You should probably try your question on math.stackexchange.com –  Uwe Franz Jun 8 '13 at 11:17
I tried there but have not got any answer –  aaaaaa Jun 9 '13 at 3:56

The answer is yes. A good version of the ergodic theorem for Markov Chains says that for any discrete Markov chain, $\frac1{n}\sum_{k=0}^{n-1} 1_{\{X_k=j\}}$ almost surely converges to $\frac{1_{T_j<+\infty}}{E^j[T_j]}$. So, by dominated convergence, the quantity that you consider converges to
$\frac{P^j(T_j<+\infty)}{E^j[T_j]}$. If the limit is positive, $E^j[T_j]<+\infty$, which is precisely positive recurrence.