
a.
Prove that joint density
a.

Explanation of Solution
Calculation:
In Exercise 5.65 the joint density is given as follows:
f(y1,y2)={[1−α{(1−2e−y1)(1−2e−y1)}]e−y1−y2, y1≥0,y2≥00 , otherwise,
Where marginal densities of Y1 and Y2 follow exponential distribution with mean 1 and −1≤α≤1.
It is known that the cumulative density function of Y1 is F1(y1)=1−e−y1 and the cumulative density function of Y2 is F2(y2)=1−e−y2.
Now, substitute F1(y1)=1−e−y1 and F2(y2)=1−e−y2 in the given joint cumulative density function.
That is,
F(y1,y2)=(1−e−y1)(1−e−y2)[1−α{1−(1−e−y2)}{1−(1−e−y2)}]=(1−e−y1)(1−e−y2)[1−αe−y1e−y2]
Hence, the joint probability density function is obtained as follows:
f(y1,y2)=∂2F(y1,y2)∂y1∂y2=∂2[(1−e−y1)(1−e−y2)[1−αe−y1e−y2]]∂y1∂y2=∂2[(1−e−y1)(1−e−y2)]∂y1∂y2−α∂2[(1−e−y1)(1−e−y2)e−y1e−y2]∂y1∂y2=e−y1e−y2−e−y1e−y2{α(1−2e−y1)(1−2e−y2)}=e−y1−y2−e−y1−y2{α(1−2e−y1)(1−2e−y2)}=[1−α{(1−2e−y1)(1−2e−y1)}]e−y1−y2
Hence, it is proved that the joint density function of Y1 and Y2 is same as given in Exercise 5.65.
b.
Evaluate F(y1,y2) for any α where −1≤α≤1.
b.

Answer to Problem 163SE
The joint cumulative density function of Y1 and Y2 is F(y1,y2)=y1y2[1−α(1−y1)(1−y2)], −1≤α≤1.
Explanation of Solution
Calculation:
Consider that Y1 and Y2 follow Uniform distribution over the interval (0,1).
Hence, the F1(y1)=y1 and F2(y2)=y2.
Now, substitute F1(y1)=y1 and F2(y2)=y2 in the given joint cumulative density function.
That is,
F(y1,y2)=(y1)(y2)[1−α{1−(y1)}{1−(y2)}]=y1y2[1−α(1−y1)(1−y2)]
Hence, the joint cumulative density function of Y1 and Y2 is F(y1,y2)=y1y2[1−α(1−y1)(1−y2)], −1≤α≤1.
c.
Obtain the joint density function associated with the distribution function that is obtained in Part (b).
c.

Answer to Problem 163SE
The joint density function is,
f(y1,y2)={1−α(1−2y1)(1−2y2), 0≤y1≤1,0≤y2≤10, Otherwise.
Explanation of Solution
From Part (b), the joint cumulative density function of Y1 and Y2 is obtained as F(y1,y2)=y1y2[1−α(1−y1)(1−y2)], −1≤α≤1.
Hence, the joint probability density function is obtained as follows:
f(y1,y2)=∂2F(y1,y2)∂y1∂y2=∂2[y1y2[1−α(1−y1)(1−y2)]]∂y1∂y2=∂2(y1y2)∂y1∂y2−α∂2[y1y2(1−y1)(1−y2)]∂y1∂y2=1−α∂2[(y1−y21)(y2−y22)]∂y1∂y2=1−α(1−2y1)(1−2y2)
Thus, the joint density function is,
f(y1,y2)={1−α(1−2y1)(1−2y2), 0≤y1≤1,0≤y2≤10, Otherwise.
d.
Provide two specific and different joint densities that yield marginal densities for Y1 and Y2 both of which are Uniform over the interval (0,1).
d.

Explanation of Solution
Calculation:
From Part (c), the density function is obtained as follows:
f(y1,y2)={1−α(1−2y1)(1−2y2), 0≤y1≤1,0≤y2≤10, Otherwise,
Where marginal densities of Y1 and Y2 follow Uniform over the interval (0,1).
The marginal density function does not depend on the values of α.
Thus, as α∈(0,1), one can take any two values of α within that range to obtain two different densities.
Consider α=−0.5 and substitute α=−0.5 in the given probability density function.
f(y1,y2)={1+0.5(1−2y1)(1−2y2), 0≤y1≤1,0≤y2≤10, Otherwise
Consider α=0.5 and substitute α=0.5 in the given probability density function.
f(y1,y2)={1−0.5(1−2y1)(1−2y2), 0≤y1≤1,0≤y2≤10, Otherwise
Hence, these two specific and different joint densities yield marginal densities for Y1 and Y2 that are both Uniform over the interval (0,1).
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Chapter 5 Solutions
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