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Suppose that on each play of the game a gambler either wins 1 with probability \(p\) or loses 1 with probability \(1-p .\) The gambler continues betting until she or he is either winning \(n\) or losing \(m\). What is the probability that the gambler quits a winner?

Short Answer

Expert verified
The probability that the gambler quits the game as a winner is: \(P(0) = \dfrac{1}{p}\)

Step by step solution

01

Understand the random walk situation

This problem can be considered as a random walk in one dimension, where the gambler starts at position 0 and moves forward (win) or backward (lose) in each step. The game ends when the gambler reaches position \(n\) (winning) or \(-m\) (losing).
02

Set up the equation for probability

Let's denote the probability of the gambler reaching position \(k\) as \(P(k)\). Since the gambler can only win with probability \(p\) and lose with probability \((1-p)\), we can write the equation for the probability of the gambler reaching a given position as: \(P(k) = p \cdot P(k-1) + (1-p) \cdot P(k+1)\) We can rewrite this equation by isolating the term \(P(k-1)\): \(P(k-1) = \dfrac{P(k) - (1-p) \cdot P(k+1)}{p}\)
03

Determine the boundary conditions for the random walk process

In this problem, we have two boundary conditions that must be satisfied. The first is when the gambler reaches the winning threshold (\(n\)), and the second is when the gambler reaches the losing threshold (\(-m\)). We can express these boundary conditions as: \(P(n) = 1\) (probability of winning when reaching \(n\)) \(P(-m) = 0\) (probability of winning when reaching \(-m\))
04

Identify the required probability

We are interested in finding the probability \(P(0)\), which represents the probability that the gambler quits the game as a winner, starting from the initial position.
05

Set up the recurrence relation

From Step 2, we have the recurrence relation: \(P(k-1) = \dfrac{P(k) - (1-p) \cdot P(k+1)}{p}\) We can express \(P(0)\) using this recurrence relation: \(P(0) = \dfrac{P(1) - (1-p) \cdot P(2)}{p}\)
06

Use the boundary conditions to solve the recurrence relation

Using the boundary conditions \(P(n)=1\) and \(P(-m)=0\), we can iteratively substitute the recurrence relation obtained in Step 5. Each time we apply the relation, we will decrease the index of \(P\) by 1, and we will reach the required probability \(P(0)\) after doing this \(m\) times: \(P(0) = \dfrac{P(1) - (1-p) \cdot P(2)}{p} = \dfrac{ \dfrac{P(2) - (1-p) \cdot P(3)}{p} - (1-p)\cdot P(2)}{p} = \dots\) Continue to apply the recurrence relation \(m\) times to reach \(P(0)\): \(P(0) = \dfrac{P(m) - (1-p) \cdot P(m+1)}{p} = \dfrac{1 - (1-p) \cdot 0}{p} = \dfrac{1}{p}\) Thus, the probability that the gambler quits the game as a winner is: \(P(0) = \dfrac{1}{p}\)

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Probability Models
In the realm of probability, making predictions about uncertain events is crucial. A probability model is a mathematical representation that helps us describe real-world random processes. It assigns probabilities to different possible outcomes. These models can be discrete, with countable outcomes, or continuous, with infinite possibilities.
In this particular scenario, the gambler's situation can be seen as a stochastic process. Each play of the game represents a move in a random walk. The random walk is a type of probability model where the outcome of each step depends on chance. In simple terms, the gambler either wins or loses a unit on each turn, with probabilities of winning as \( p \) and losing as \( 1-p \).
Probability models, such as this one, can be utilized to predict the likelihood of various outcomes in uncertain environments. They are foundational in understanding processes similar to stock market fluctuations or predicting weather changes.
Gambler's Ruin
The Gambler's Ruin is a classic problem in probability theory concerning a gambling scenario. It explores the fate of a gambler who relentlessly attempts to reach their desired winnings, risking complete loss in the process. The key question is whether the gambler will end up with the target winnings before depleting their funds.
This notion is demonstrated in the exercise where the gambler continues until reaching a certain winning amount \( n \) or losing \( m \). It's an excellent example of how repeated betting can lead to eventual ruin or success. The probability calculated provides the answer to whether the gambler will quit as a winner.
The relevance of the Gambler's Ruin extends beyond monetary betting. It's also applicable in real-world phenomena like population risks or even in modeling the burnout in resource allocations. Understanding this concept helps in strategizing decisions where outcomes are highly unpredictable.
Boundary Conditions
In solving probability problems, especially those involving random walks, boundary conditions play a pivotal role. They are specific requirements that must be satisfied for the scenario being considered. Essentially, they serve as markers for the limits of the random experiment.
In the gambler's context, the boundaries are set at the points of winning \( n \) and losing \( -m \). These conditions state that once the gambler reaches these respective points, the game comes to an end. As such, we have \( P(n) = 1 \) and \( P(-m) = 0 \).
The essence of boundary conditions lies in their ability to define where and how a process can terminate. Properly identifying and applying them is crucial for solving and understanding random walks and similar probability scenarios. They allow for deriving meaningful solutions regarding the probabilities of various outcomes.
Recurrence Relations
Recurrence relations are equations that define sequences or multi-stage processes in terms of their previous terms. They are fundamental in breaking down complex problems into simpler, more manageable steps. In probability, they provide valuable insights by linking the probability of certain outcomes over consecutive steps.
In this exercise, the recurrence relation \( P(k-1) = \frac{P(k) - (1-p) \cdot P(k+1)}{p} \) helps determine the probability at each stage of the random walk. It serves as the backbone for calculating \( P(0) \), the likelihood of the gambler ultimately quitting as a winner.
By employing recurrence relations, one can methodically solve for unknown probabilities using known boundary conditions. This makes them an essential tool for tackling sequential processes in mathematics and understanding the dynamics that govern them.

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Most popular questions from this chapter

A professor continually gives exams to her students. She can give three possible types of exams, and her class is graded as either having done well or badly. Let \(p_{i}\) denote the probability that the class does well on a type \(l\) exam, and suppose that \(p_{1}=0.3, p_{2}=0.6\), and \(p_{3}=0.9 .\) If the class does well on an exam, then the next exam is equally likely to be any of the three types. If the class does badly, then the next exam is always type 1. What proportion of exams are type \(i, i=1,2,3 ?\)

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