Questions tagged [probability-generating-fn]

A probability generating function is a function defined as a power series which contain all the probability mass function values of a discrete probability distribution. It is related to the moment generating function, and also known as a z-transform.

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Probability generating function and binomial coefficients

I'm reading an article where the authors derive the mass function of a compound distribution by considering the generating function. The generating function of interest for a random variable $N$ is a ...
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Difficulties with the PGF of X+Y with Y~Poisson(1) and X~Poisson(Y)

The pair of random variables $(X, Y )$ is distributed as follows. $Y$ has probability mass function $\text{Poisson}(1).$ Given $Y , X$ has probability mass function $\text{Poisson}(Y ).$ Show that the ...
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Probability Generating Function for The Difference of Two Binomially Distributed Random Variables?

Suppose I have 2 random variables: $X\sim \textrm{Bin}(m,p_1)$ and $Y\sim \textrm{Bin}(n,p_2).$ I want to find the distribution of $S=X-Y$ using the probability generating function ($PGF$) treating $S$...
Wind Fish's user avatar
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Generating Function for Coin Flip followed by a roll of two dice

I'm self studying probability, specifically generating functions. I'm searching for the relation between conditional probabilities and probably generating functions in order to justify writing the ...
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Average cost of getting a specific card from deck of 9?

In a game, I am looking to draw the hero card out of 9 possible cards. The first card (full deck) costs 300 gems. All subsequent draws cost 600 gems. I can either keep drawing one card at a time till ...
Dro's user avatar
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PGF of sum of RVs is a composition of PGFs

Let $\\\{X_n\\\}$ be a sequence of i.i.d. random variables whose values are non-negative integers. Let $N$ be a random variable that is independent of $\\\{X_n\\\}$. $N$'s values are also non-negative ...
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Find probability generating function(p.g.f) of a compound distribution

I have a discrete compound distribution for a random variable: $$S_N = X_1 + X_2 +\dots + X_N,$$ where $X_1, X_2, \dots, X_N$ are IID count random variables, and $N$ is a count random variable too. ...
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Generating function of a random walk

Consider a random walk with $S_n=\sum^n_{i=1}X_i$, where the random i.i.d. steps $X_i$ take values $-1,0,2$ with probabilities $1/9,1/9,7/9$ respectively. Set $S_0=1$. I would like to calculate the ...
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Likelihood loss function for finite support probability distribution in Neural Networks

I have managed to reproduce solution from this article and made it work for my dataset. Instead of making a Neural Network output a scalar (regression), we make it output two parameters of a ...
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Question on Probability generating function

For a discrete random variable $X$ with the PGF P(z). I have been given $Y = 2X$ and I need to compute the PGF, G(Z) of this new random variable. We know that the probability generating function is $E(...
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A Skip Free Negative Random Walk

Suppose $\{X_{n}|n\geq 1\}$ is independent, identically distributed distribuited. Define $S_{0}=X_{0}=1$ and for $n\geq 1$ $$S_{n}=X_{0}+X_{1}+\cdots+X_{n}.$$ For $n\geq 1$ the distribution of $X_{n}$ ...
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Extreme birthday problem

I have an extreme version of the birthday problem. I want to know: The probability that $m$ individuals will share a birthday The expected $m$ given the number of individuals The slight complication ...
Anonymous Scientist's user avatar
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Why do we use exponent in characteristic function? [closed]

A student who is attending probability 101, learned about normal distribution and generating functions recently. We are given a "generating function" as follows: $$G(t)=\langle e^{itx}\...
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How do I analytically calculate variance of a recursive random variable?

Suppose I have a chest. When you open the chest, there is a 60% chance of getting a prize and a 40% chance of getting 2 more chests. Let $X$ be the number of prizes you get. What is its variance? ...
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Branching process Galton Watson

Using the Galton Watson branching process Assume that a fox had 0,1,2,3 offspring with probabilities p0,p1,p2,p3 respectively. find the probability distribution for G1 and G2. I worked out G1 ...
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Can the pdf of the difference of two independent random variables be found out using their Probability Generation Functions?

I recently learned of probability generation functions and that the sum of two independent random variables can be found out by multiplying the PGFs. I wanted to know if anything similar can be done ...
Ishan Kashyap Hazarika's user avatar
1 vote
1 answer
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Probability generating function of mixture of discrete random variables

Consider a discrete distribution $X$ that is a mixture of two discrete distributions $A$ and $B$. Explicitly, $X=A$ with probability $p$ and $X=B$ with probability $1-p$. Denote the pgfs of $A$ ...
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Probability Generating Functions: How to use them?

For a discrete variable $X$ that takes on nonnegative integer values $\{0,1,2,\ldots\}$, the probability generating function is defined as $$G(s) = \sum_{k=0}^\infty P(X=k) s^k$$ It is easy to show ...
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In general, how should we find the pmf given only the moment generating function without comparing its form to that of famous pmf?

Background It is known that moment generating function generates moments, but does it hold information about the probability of the random variable being realised at a particular value? Example ...
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Fit data based on generating function

Suppose I have iid data generated from a discrete random variable $X_i \sim D(\lambda)$, and I would like to infer the parameter $\lambda$. Unfortunately, I do not know the likelihood function for $D$,...
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Generating Function for sum of N dice [or other multinomial distribution] where lowest N values are "dropped" or removed

Background I found this interesting question Formula for dropping dice (non-brute force) and excellent answer https://stats.stackexchange.com/a/242857/221422, but couldn't figure out how to ...
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Stopping condition for MCTS

I'm trying to come up with a stopping condition for MCTS with exploration. Say we have $m$ actions at the root state, and after $N$ playouts they have $n_1\geq ...\geq n_m$ visits. I want to test the ...
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Stochastic Processes: Randomly Stopped Sums *vs* the sum of I.I.D. Random Variables? (For Finding PGF's)

So I'm studying Stochastic Processes (specifically PGF's). I see two definitions but I don't seem to know how to differentiate between them properly. Theorem 1 Suppose $X_1, ... , X_n$ are ...
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Using Chebyshev's inequality to obtain lower bounds

Let $X_1$ and $X_2$ be i.i.d. continuous random variables with pdf $f(x) = 6x(1-x), 0<x<1$ and $0$, otherwise. Using Chebyshev's inequality, find the lower bound of $P\left(|X_1 + X_2-1| \le\...
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Can a $\require{enclose}\enclose{horizontalstrike}{z-transform}$ Probability Generating Function be used to solve this recurrence relation?

Imagine a simple random walk where $x_n=\sum_{i=0}^ny_i$ and $y_i = \begin{cases} 1, & \text{with prob 1/2} \\ -1, & \text{with prob 1/2} \end{cases}$ Then say we want to find the ...
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What is the distribution of a Poisson variable, where the Poisson rate is Normal (or Binomial)?

What is the distribution of $X$ if $$ X \sim \text{Poisson}(\lambda), \quad \text{where }\lambda \sim N(\mu,\sigma^2)$$ or $$ X \sim \text{Poisson} (\lambda), \quad \text{where }\lambda \sim ...
Ian Sudbery's user avatar
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Probability Generating Functions and Discrete Time Queuing Chains

I've been learning about Markov Chains for a reading course I'm doing and I've become stuck on some of the notes I was given. It defines a discrete time queuing chain for a server by $$X_{t+1}=\begin{...
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3 votes
1 answer
1k views

Compound Poisson Process, probability generating function

If we have a iid random variables $X_i$ with probability generating function $\xi(t) = E[t^{x_i}]$ and $N$ is Poisson with mean $\lambda$ with Probability generating function : $$ \begin{aligned} \phi(...
rannoudanames's user avatar
5 votes
2 answers
723 views

PMF of the number of trials required for two successive heads

A coin with probability $p$ of landing a head is tossed repeatedly till the occurrence of two consecutive heads. Let $X$ be the random variable denoting the the number of trials needed. What is the ...
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For independent RVs $X_1,X_2,X_3$, does $X_1+X_2\stackrel{d}{=}X_1+X_3$ imply $X_2\stackrel{d}{=}X_3$?

Let $X_1,X_2$, and $X_3$ be independent random variables such that $X_1+X_2$ and $X_1+X_3$ have the same distribution. Does it follow that $X_2$ and $X_3$ have the same distribution? Can this be ...
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Formula for dropping 2 dice (non-brute force)

This is a follow up from my previous question: Formula for dropping dice (non-brute force) which asked how to determine the statistics for dropping the lowest die but now I'm asking how to determine ...
SkySpiral7's user avatar
2 votes
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560 views

Probability Generating Function for Geometric using conditioning

I was wondering if someone could look over my derivation of the probability generating function for geometric RV. I think I arrived at the right conclusions. I wanted to take the alternate route over ...
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1 answer
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What is the meaning of a probability generating function given domain T?

What does it mean when I plug in $0, 1, ..., n$ into a probability generating function? What does a probability generating function tell me about the number I plugged in?
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Poisson binomial distribution-like problem

Given n trials, where, on each trial, you have a given probability of either winning or losing a set amount of money (with both the amount of money and the probability changing for each trial)- what ...
user2346333's user avatar
1 vote
1 answer
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What would we obtain if we set any value other than 0 or 1 to $s$ in $G_X(s)$?

Let $X$ be a discrete RV with PGF $G_X(s) = \frac{s \cdot (2 + 3s^2)}{5}$ . Find the distribution of $X$. Here, if we continue to differentiate and set $s=0$ in $G_X$, we obtain $P(X=0,1,2,… … …)$. ...
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1 vote
1 answer
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If $G_X$ is a Probability Generating Function of $X$, what does it mean by $G_X(0)=\mathbb P(X=0)$ and $G_X(1)=1$?

Let $G_X$ is a Probability Generating Function of $X$. What does it mean by $G_X(0)=\mathbb P(X=0)$ ? $G_X(1)=1$ ? Why are these identities important?
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2 votes
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Generating from an unknown discrete posterior distribution

My question is that if I have an informative prior with density $$f(N) \propto \frac{1}{N^2}$$ and posterior for N is $$ P(N|T) \propto \big\{{n^A \choose T} {N-n^A \choose n^B-T}/{N \choose n^B}\big\...
Sarah's user avatar
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When is a generating function a probability generating function?

Given a generating function, say $G_1(z) = \frac{1}{1-z}$ it can be verified it is not a probability generating function by inspection that the sum of the probabilities (if it were a pgf) is not 100% ...
sheppa28's user avatar
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4 votes
2 answers
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MGF of Poisson Z=X+2Y

If $X\sim P(2)$ and $Y \sim P(3)$ using the moment generating function, what kind of distribution has random variable $Z=X+2Y$. So far as I know : $$ M_X(t)=e^{-\lambda(1-e^t)}=e^{-2(1-e^t)} $$ $$ ...
Cherryl's user avatar
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1 answer
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Negative probabilities - what are the two ordinary pgfs that correspond to the gf of a half-coin?

In Half of a Coin: Negative Probabilities, author considers pgf of a fair coin represented by random variable, $X = 1_H$: $$G_X(z) = E[z^X] = \sum_{x=0,1} z^xP(X=x) = (z^0)(1/2) + (z^1)(1/2) = \frac{...
BCLC's user avatar
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-1 votes
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How to define distribution, given probability generating function?

Given some probability generating function $G_X:(-1,1) \rightarrow \mathbb{R}$, Since $$P_X(k)=\frac{G^{(k)}(0)}{k!}$$ then how does one define the distribution $P_X(k)$ in practice? The difficulty ...
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Whats is the probability generating function of Poisson varied with parameter lambda

I have a question which says: "Show that the probability generating function of Poisson varied with parameter $\lambda$ is $e^{-\lambda(1-t)}$. I am not quite sure about the question because the ...
Arsalan Farooq's user avatar
3 votes
3 answers
1k views

How to calculate the sum or difference of two probability generating functions?

Suppose I have probability generating functions $$G_{X}(t) = 0.1t+0.2t^{2}+0.7t^{3}\quad\text{and}\quad G_{Y}(t)=0.5+0.4t^{2}+0.1t^{3}.$$ In other words, the random variable $X$ gets the discrete ...
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2 answers
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Proof of Algebraic Formula for the Sum of Two-Dice Toss as a Convolution

To figure out exactly the expected frequency of a given sum in a dice toss (given a certain number of dice and sides/dice), the following formula is posted here by @Glen_b (adapted to dice of six ...
Antoni Parellada's user avatar
2 votes
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Generating function from the infinitesimal generator of a continuous-time markov chain?

Summary: The basic goal is to find the time evolution of the probability generating function (or the moment generating function or the characteristic function if you prefer) for a continuous-time ...
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6 votes
3 answers
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Probability generating function for negative values of random variables?

What if we have negative integral values for a random variable?Then is it possible to write a probability generating function for it? All definitions I have seen so far is for non negative integer ...
Heisenberg's user avatar