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f X Y ( t , u ) = 4 t ( 1 - u ) for 0 t 1 , 0 u 1 (see Exercise 12 from "Problems on Random Vectors and Joint Distributions").

From the solution for Exercise 12 from "Problems on Random Vectors and Joint Distributions" we have

f X ( t ) = 2 t , 0 t 1 , f Y ( u ) = 2 ( 1 - u ) , 0 u 1 , f X Y = f X f Y

so the pair is independent.

tuappr Enter matrix [a b]of X-range endpoints [0 1] Enter matrix [c d]of Y-range endpoints [0 1] Enter number of X approximation points 100Enter number of Y approximation points 100 Enter expression for joint density 4*t.*(1-u)Use array operations on X, Y, PX, PY, t, u, and Pitest Enter matrix of joint probabilities PThe pair {X,Y} is independent
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f X Y ( t , u ) = 1 8 ( t + u ) for 0 t 2 , 0 u 2 (see Exercise 13 from "Problems on Random Vectors and Joint Distributions").

From the solution of Exercise 13 from "Problems on Random Vectors and Joint Distributions" we have

f X ( t ) = f Y ( t ) = 1 4 ( t + 1 ) , 0 t 2

so f X Y f X f Y which implies the pair is not independent.

tuappr Enter matrix [a b]of X-range endpoints [0 2] Enter matrix [c d]of Y-range endpoints [0 2] Enter number of X approximation points 100Enter number of Y approximation points 100 Enter expression for joint density (1/8)*(t+u)Use array operations on X, Y, PX, PY, t, u, and P itestEnter matrix of joint probabilities P The pair {X,Y} is NOT independentTo see where the product rule fails, call for D
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f X Y ( t , u ) = 4 u e - 2 t for 0 t , 0 u 1 (see Exercise 14 from "Problems on Random Vectors and Joint Distributions").

From the solution for Exercise 14 from "Problems on Random Vectors and Joint Distribution" we have

f X ( t ) = 2 e - 2 t , 0 t , f Y ( u ) = 2 u , 0 u 1

so that f X Y = f X f Y and the pair is independent.

tuappr Enter matrix [a b]of X-range endpoints [0 5] Enter matrix [c d]of Y-range endpoints [0 1] Enter number of X approximation points 500Enter number of Y approximation points 100 Enter expression for joint density 4*u.*exp(-2*t)Use array operations on X, Y, PX, PY, t, u, and P itestEnter matrix of joint probabilities P The pair {X,Y} is independent % Product rule holds to within 10^{-9}
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f X Y ( t , u ) = 12 t 2 u on the parallelogram with vertices ( - 1 , 0 ) , ( 0 , 0 ) , ( 1 , 1 ) , ( 0 , 1 )

(see Exercise 16 from "Problems on Random Vectors and Joint Distributions").

Not independent by the rectangle test.

tuappr Enter matrix [a b]of X-range endpoints [-1 1] Enter matrix [c d]of Y-range endpoints [0 1] Enter number of X approximation points 200Enter number of Y approximation points 100 Enter expression for joint density 12*t.^2.*u.*(u<=min(t+1,1)).* ... (u>=max(0,t)) Use array operations on X, Y, PX, PY, t, u, and Pitest Enter matrix of joint probabilities PThe pair {X,Y} is NOT independent To see where the product rule fails, call for D
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f X Y ( t , u ) = 24 11 t u for 0 t 2 , 0 u min { 1 , 2 - t } (see Exercise 17 from "Problems on Random Vectors and Joint Distributions").

By the rectangle test, the pair is not independent.

tuappr Enter matrix [a b]of X-range endpoints [0 2] Enter matrix [c d]of Y-range endpoints [0 1] Enter number of X approximation points 200Enter number of Y approximation points 100 Enter expression for joint density (24/11)*t.*u.*(u<=min(1,2-t)) Use array operations on X, Y, PX, PY, t, u, and Pitest Enter matrix of joint probabilities PThe pair {X,Y} is NOT independent To see where the product rule fails, call for D
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Source:  OpenStax, Applied probability. OpenStax CNX. Aug 31, 2009 Download for free at http://cnx.org/content/col10708/1.6
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