Some states of a Markov chain are traps: once entered, the chain stays forever. A gambler who reaches \$0 or a target stops playing. Two questions follow: where does the chain end up, and how long does it take? Conditioning on the first step turns each question into a small linear system, solved as in elimination.
Idea clave
A state $j$ is absorbing if $P_{jj} = 1$. Let $h_i$ be the chance of ending in a chosen absorbing state starting from $i$, and $m_i$ the expected number of steps to absorption. Then $h = 1$ in the chosen state, $h = 0$ in the other absorbing states, $m = 0$ in all absorbing states, and for every other state $$h_i = \sum_j P_{ij}h_j, \qquad m_i = 1 + \sum_j P_{ij}m_j.$$
Por qué funciona
Condition on the first step. From $i$ the chain moves to $j$ with probability $P_{ij}$, and by the Markov property it then behaves as if it had started at $j$. So the chance of ending in the target is the weighted average $\sum_j P_{ij}h_j$ (total probability). For the time, the step just taken counts 1, and then the expected remaining time from $j$ is $m_j$: $m_i = 1 + \sum_j P_{ij}m_j$. Absorbing states supply the boundary values.
Worked examples
Ejemplo resuelto: Gambler's ruin
A gambler with \$$k$ bets \$1 at a time on fair coin flips until reaching \$$N$ or \$0. Find the chance of reaching \$$N$.
- $h_0 = 0$, $h_N = 1$, and for $0 < i < N$: $h_i = \frac{1}{2}h_{i - 1} + \frac{1}{2}h_{i + 1}$.
- Rearranged: $h_{i + 1} - h_i = h_i - h_{i - 1}$. The steps are equal, so the $h_i$ rise in a straight line from 0 to 1.
- $h_k = \frac{k}{N}$; with $k = 1$ and $N = 4$, the chance is $\frac{1}{4}$.
Respuesta: $\frac{k}{N}$
Ejemplo resuelto: How long it lasts
For the same game, find the expected number of bets.
- $m_0 = m_N = 0$ and $m_i = 1 + \frac{1}{2}m_{i - 1} + \frac{1}{2}m_{i + 1}$.
- Try $m_i = i(N - i)$: $$1 + \frac{1}{2}(i - 1)(N - i + 1) + \frac{1}{2}(i + 1)(N - i - 1) = i(N - i)$$ It fits the equations and the boundary values.
- From \$1 with $N = 4$: $1 \cdot 3 = 3$ bets on average. From \$2: $2 \cdot 2 = 4$.
Respuesta: $k(N - k)$
Ejemplo resuelto: Waiting for two heads in a row
Flip a fair coin until HH appears. Find the expected number of flips.
- States: "start" and "just flipped H". Let $m_0, m_1$ be the expected flips still needed.
- $m_0 = 1 + \frac{1}{2}m_1 + \frac{1}{2}m_0$ and $m_1 = 1 + \frac{1}{2} \cdot 0 + \frac{1}{2}m_0$ (a T sends you back to the start).
- The first gives $m_0 = 2 + m_1$; then $m_1 = 1 + \frac{1}{2}(2 + m_1)$, so $m_1 = 4$ and $m_0 = 6$.
Respuesta: 6 flips
With an unfair coin, winning with probability $p$, the steps $h_{i + 1} - h_i$ grow by the factor $r = \frac{1 - p}{p}$ each time, and $h_k = \frac{1 - r^{k}}{1 - r^{N}}$ (for $p \ne \frac{1}{2}$).
Error común
Forgetting to add 1 for the step just taken when writing the equations for expected times. Without it, $m_i = \sum P_{ij}m_j$ is solved by $m = 0$ everywhere, which is plainly wrong. Check: absorption probabilities have no "1 +" and lie between 0 and 1; expected times have "1 +" in every equation of a non-absorbing state.