# Category Archives: Max and Mins

## Minimum Dice Roll

Suppose that three fair 6-sided dice are rolled.

1. Let $$M$$ be the minimum of three numbers rolled. Find $$\mathbb{E}(M)$$.
2. Let $$S$$ be the sum of the largest two rolls. Find $$\mathbb{E}(S)$$.

## Difference between max and min

Let $$U_1,U_2,U_3,U_4,U_5$$ be independent, each with uiform distribution on $$(0,1)$$. Let $$R$$ be the distance between the max and the min of the $$U_i$$’s. Find

1. $$\mathbf{E} R$$
2. the joint density of the max and the min of the $$U_i$$’s.
3. the $$\mathbf{P}(R> .5)$$

[pitman p355, #14]

## Point of increase

Suppose $$U_1,U_2, …$$ are independent uniform $$(0,1)$$ random variables. Let $$N$$ be the first point of increase. That is to say the first $$n \geq 2$$ such that $$U_n > U_{n-1}$$. Show that for $$u \in (0,1)$$:

1. $\mathbf{P}(U_1 \leq u \ { and } \ N=n)= \frac{u^{n-1}}{(n-1)!}-\frac{u^{n}}{n!} \quad;\quad n \geq 2$
2. $$\mathbf{E}(N)=e$$

Some useful observations:

• $\mathbf{P}(U_1 \leq u \ { and } \ N=n) = \mathbf{P}(U_1 \leq u \ { and } \ N \geq n) -\mathbf{P}(U_1 \leq u \ { and } \ N \geq n+1)$
• The following events are equal
$\{U_1 \leq u \quad{ and } \quad N \geq n\} = \{U_{n-1}\leq U_{n-2} \leq \cdots \leq U_2\leq U_{1} \leq u \}$
• $\mathbf{P}\{U_2 \leq U_1 \leq u \}= \int_0^u \int_0^{u_1} du_2 du_1$

## An example of min and change of variable

Suppose $$R_1$$ and $$R_2$$ are two independent random variables with the same density function

$f(x)=x\exp(-{\textstyle \frac12 }x^2)$

for $$x\geq 0$$. Find

1. the density of $$Y=\min(R_1,R_2)$$;
2. the density of $$Y^2$$

[Pitman p. 336 #21]

## Min, Max, and Exponential

Let $$X_1$$ and $$X_2$$ be random variables and let $$M=\mathrm{max}(X_1,X_2)$$ and $$N=\mathrm{min}(X_1,X_2)$$.

1. Argue that the event $$\{ M \leq x\}$$ is the same as the event   $$\{X_1 \leq x, X_2 \leq x\}$$ and similarly that t the event $$\{ N > x\}$$ is the same as the event   $$\{X_1 > x, X_2 > x\}$$.
2. Now assume that the $$X_1$$ and $$X_2$$ are independent and distributed with c.d.f. $$F_1(x)$$ and $$F_2(x)$$ respectively . Find the c.d.f. of $$M$$ and the c.d.f. of $$N$$ using the proceeding observation.
3. Now assume that $$X_1$$ and $$X_2$$ are independently and exponentially  distributed with parameters $$\lambda_1$$ and $$\lambda_2$$ respectively. Show that $$N$$ is distributed exponentially and identify the parameter  in the exponential distribution of $$N$$.
4. The route to a certain remote island contains 4 bridges. If the time to collapse of each bridge is exponential distributed with mean 20 years and is independent of the other bridges, what is the distribution of the time until the road is impassable because one of the  bridges has collapsed.

[Jonathan Mattingly]

## Order statistics II

Suppose $$X_1, … , X_{17}$$ are iid uniform on $$(.5,.8)$$. What is $${\mathbf{E}} [X_{(k)}]$$ ?

## Order statistics I

Suppose $$X_1, … , X_n \stackrel{iid}{\sim} U(0,1)$$. How large must $$n$$ be to have that $${\mathbf{P}}(X_{(n)} \geq .95) \geq 1/2$$ ?

## Joint of min and max

Let $$X_1,…,X_n \stackrel{iid}{\sim} \mbox{Exp}(\lambda)$$

Let $$V = \mbox{min}(X_1,…,X_n)$$ and  $$W = \mbox{max}(X_1,…,X_n)$$.

What is the joint distribution of $$V,W$$. Are they independent ?

## Max and Min’s

Let $$X_1,\cdots, X_n$$ be random variables which are i.i.d. $$\text{unifom}(0,1)$$. Let $$X_{(1)},\cdots, X_{(n)}$$ be the associated order statistics.

1. Find the distribution of $$X_{(n/2)}$$ when $$n$$ is even.
2. Find $$\mathbf{E} [ X_{(n)} – X_{(1)} ]$$.
3. Find the distribution of $$R=X_{(n)} – X_{(1)}$$.

## Expectation of min of exponentials

There are $$15$$ stock brokers. The returns (in thousands of dollars) on each brokers is modeled as a separate independent exponential distribution $$X_1 \sim \mbox{Exp}(\lambda_1),…,X_{15} \sim \mbox{Exp}(\lambda_{15})$$. Define $$Z = \min\{X_1,…,X_{15}\}$$.

What is $$\mathbf{E}(Z)$$ ?

## Maximum of die rolls

Let $$X_1,…,X_5$$ be five iid rolls of six sided die. Let $$Z = \mbox{max}\{X_1,…,X_5\}$$. Compute $$\mathbf{E}(Z)$$.

## Two die

Two dice are rolled. Find the probabilities of the following events.

a) the maximum of the two numbers rolled is less than or equal to 2;

b) the maxinum of the two numbers rolled is less than or equal to 3;

c) the maximum of the two numbers rolled is exactly equal to 3;

d) Repeat b) and c) with  3 replaced by $$x=1,…,6$$;

e) Denote $$\mathbf{P}(x)$$ as the probability that the maximum number is exactly $$x$$.

Compute  $$\sum_{x=1}^6\mathbf{P}(x)$$.

[Pitman Page 10, #7]

## Dice rolls: Explicit calculation of max/min

Let $$X_1$$ and $$X_2$$ be the number obtained on two rolls of a fair die. Let $$Y_1=\max(X_1,X_2)$$ and $$Y_2=\min(X_1,X_2)$$.

1. Display the joint distribution tables for $$(X_1,X_2)$$.
2. Display the joint distribution tables for $$(Y_1,Y_2)$$.
3. Find the distribution of $$X_1X_2$$.

Combination of [Pitman, p. 159 #4 and #5]

## n – Dice : Max and Min

Suppose that a die has $$n$$ sides. Compute the probability that:

1.  the maximum of the two numbers rolled is less than or equal to 2;
2. the maximum of the two numbers rolled is less than or equal to $$i \in \{1,\dots, n\}$$;
3. the maximum of the two numbers rolled is equal to $$i \in \{1,\dots,n\}$$.

## Dice Rolls – Max and Min

Suppose two 4-sided dice are rolled. Find the probabilities of the following events:

1. the maximum of the two numbers rolled is less than or equal to 2;
2. the maximum of the two numbers rolled is less than or equal to 3;
3. the maximum of the two numbers rolled is equal to 3;
4. Repeat part c for the maximum equal to 1, 2, and 4.
5. If M is the maximum of the two numbers, then

$\mathbf{P}(M = 1) + \mathbf{P}(M = 2) + \mathbf{P}(M = 3) + \mathbf{P}(M = 4) = 1$