# FEGYVERNEKI SÁNDOR, PROBABILITY THEORY AND MATHEmATICAL

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1 FEGYVERNEKI SÁNDOR, PROBABILITY THEORY AND MATHEmATICAL STATIsTICs 4

2 IV. RANDOm VECTORs 1. JOINTLY DIsTRIBUTED RANDOm VARIABLEs If are two rom variables defined on the same sample space we define the joint distribution function of for all real by for each real for each real these are the marginal distribution functions. The joint probability mass functions of is defined by Let Then, if either finite or countable we say that are jointly discrete. If there exists a nonnegative, real function that for any real then is called the joint probability density function. Two rom variables are said to be independent if their joint distribution function can be expressed as a product of the marginal distribution functions, i.e. for all real The covariance of, written is defined by If are said to be uncorrelated. The correlation coefficient by of two jointly distributed rom variables with finite positive variances is defined Although, we defined the joint distribution function for only two rom variables, the concept can be extended to any finite number of rom variables. All other concepts we have discussed are then defined just as they were for two rom vaiables. Thus, for example, one condition that means the rom variables are independent is that

3 for all real Theorem: A function is a joint distribution function iff the function is left continuous, for all real for all where the values of are Theorem: Let be independent rom variables with distribution functions respectively. Then the distribution function of (a) for each is given by (b) for each is given by Theorem: (Convolution Theorem) Let be jointly distributed rom variables with Then the following hold. (a) If are independent discrete rom variables with each taking on the values then takes on the values

4 (b) If are independent continuous rom variables then where the density function of is given by PROBLEM EXAMPLE 1 Consider a sample of size 2 drawn with replacement (without replacement) from an urn containing 2 white, 1 black two red balls. Let the rom variables be defined as follows; for or depending on whether the ball drawn on the th draw is white or nonwhite. (i) Describe the joint probability distribution of (ii) Describe the marginal probability distributions. The rom variables are clearly jointly discrete. Consequently, to describe their joint probability distribution, it suffices to state their joint probability mass function Similarly, to describe their marginal probability distributions, it suffices to describe their marginal probability mass functions These functions are conveniently presented in the following tables: Sampling with replacement Sampling without replacement

5 1 PROBLEM EXAMPLE 2 Suppose that at two points in a room one measures the intensity of sound caused by general background noise. Let be rom variables representing the intensity of sound at the two points. Suppose that the joint probability distribution of the sound intensities, is continuous, with the joint probability density function given by Find the marginal probability density functions of Further, find The marginal probability density functions are given by Note that the rom variables are identically distributed. Next, the probability that each sound intensity is less than 1 is given by The probability that the sum of the sound intensities is less than 1 is given by PROBLEM EXAMPLE 3 Suppose that the life in hours of electronic tubes of a certain type is known to be approximately normally distributed with parameters What is the probability that a rom sample of four tubes will contain no tube with a lifetime of less than 180 hours? Let denote the respective lifetimes of the four tubes in the sample. The assumption that the tubes constitute a rom sample of a rom variable normally distributed with parameters is to be interpreted as assuming that the rom variables are independent, with marginal probability density functions

6 The probability that each tube in the sample has a lifetime greater than, or equal to, 180 hours, is given by since 2. ExERCIsEs visible invisible 1. Alan Bob play a game in which Alan uses a 6-faced die Bob a 4-faced die. To make the game fair, they decide to double all Alan s scores treble Bob s. What is the expected value variance of the total score registered on the two dice? variance Bob has the advantage; Alan s variance Bob s 2. Let be independent rom variables. Prove that 3. If are independent rom single digit numbers other than zero, calculate the variance of (i) (ii) (iii) (iv) (i) (ii) (iii) (iv) 4. A man travels from his London office to his home by a tube journey from station A to station B. His walking times to A from B add up to 5 minutes with negligible variation, the variable factors in the journey being as follows, measured in minutes: (i) waiting for a train: mean time is 8, stard deviation is 2.6, (ii) train journey: mean time is 47, stard deviation is 1.8. Assuming that these two factors are independent normally distributed, find the mean time stard deviation of his whole journey. Estimate the probability of the whole journey taking (i) less than 52 minutes, (ii) more than 65 minutes, (iii) between minutes.

7 60 minutes, 3.16 minutes, (i) , (ii) 0.057, (iii) Three independent components are placed in an electrical circuit in such a way that, as soon as the first fails, the second (reserve) component is automatically switched in, then when the second component itself fails, the third is automatically switched in. The circuit functions as long as one of the components works. If the lifetimes of the components are normally distributed with mean 2000 hours stard deviation 300 hours, what is the probability that: (i) the circuit will function for more than 6,500 hours, (ii) the circuit will break down within 5,000 hours? (i) 0.168, (ii) Verify that the following functions are joint probability density functions determine the covariance: i. the joint normal distribution where ii. the joint uniform distribution on iii. the joint Pareto s distribution. (i) 7. Prove that if are jointly normally distributed rom variables whose correlation coefficient vanishes then are independent. 8. A sample of size 3 is drawn without replacement from an urn containing 8 white balls 4 black balls. For let be equal to 1 or 0, depending on whether the ball drawn on the th draw is white or black. Find Mean, variances, 0.5; covariance,

8 9. Let be jointly normally distributed with mean 0, variance 1, covariance Find 10. Consider 2 events such that Define rom variables or depending on whether the event has or has not occured, or depending on whether the event has or has not occured. Find Are independent? are uncorrelated but not independent. 11. Consider a sample of size drawn with replacement (without replacement) from an urn containing white black balls. For let or depending on whether the ball drawn on the th draw is white or nonwhite. Describe the joint probability density function of Describe the marginal distributions of Describe the distributions of the rom variables 12. Consider rom variables, with joint probability distribution function specified by the joint probability density function i. ii. iii. iv. v.

9 (i) (a) (b) (ii) (a) (b) (iii) (a) (b) (iv) (a) 0, (b) 0, (v) (a) (b) 13. The output of a certain electronic apparatus is measured at different times. Let be the observation obtained. Assume that are independent romvariables with density function Find the probability that. 14. Let be jointly continuous rom variables with a probability density function i. Are independent rom variables? ii. Are identically distributed rom variables? iii. Are normally distributed rom variables? iv. Find Hint: Use polar coordinates. v. Are independent rom variables? vi. Find vii. Find the marginal density functions of viii. Find the joint probability density function of (i) Yes, (ii) yes, (iii) yes, (iv) (v) yes, (vi) , (vii) for (viii) for 15. Consider two events such that Let the rom

10 variables be defined as or depending on whether the event has or has not occured, or depending on whether the event has or has not occured. State whether each of the following statements, is true or false: i. The rom variables are independent. ii. iii. iv. The rom variable is uniformly distributed on the interval to v. The rom variables are identically distributed. (i) True, (ii) false, (iii) true, (iv) false, (v) false. 16. Suppose are independent exponential rom variables with parameters respectively, Prove that has an exponential distribution with parameter 17. State whether the following functions are joint distribution functions or not: i. ii. (i) No, e.g. (ii) Yes. 18. Let be distribution functions let where Prove that joint distribution function its marginals are 19. Let be jointly uniformly distributed on the circle Determine the marginal density functions. if otherwise. 20. Consider a square sheet of tin, 20 inches wide, that contains 10 rows 10 columns of circular holes, each 1 inch in diameter, with centers evenly spaced at a distance 2 inches apart. i. What is the probability that a particle of s (considered as a point) blown against the tin sheet will fall upon 1 of the holes thus pass through?

11 ii. What is the probability that a ball of diameter inch thrown upon the sheet will pass through 1 of the holes without touching the tin sheet? iii. Assume an appropriate uniform distribution. 21. Let the joint density function of the rom variables be the following: Calculate the distributions of if otherwise. 22. Let the joint density function of the rom variables be the following: i. Are independent? ii. Determine the expectation variance of (i) Yes, (ii) 23. Let the rom variable be the rom variable be Bernoulli disributed with parameter Determine if are independent. 24. Let the rom variables be Determine the expectation variance of the rom variable 25. An on-line airline reservation system uses two identical duplexed computer systems, each of which has an

12 exponential time to failure with a mean of 2000 hours. Each computer system has built in redundancy so failures are rare. The system fails only if both computers fail. What is the probability that the system will not fail during one week (168 hours) of continuous operation? 30 days? , Give an example of 3 rom variables, which are independent when taken two at a time but not independent when taken together. Hint: Find such events. 27. Give an example of two rom variables which are not independent, but such that are independent. Hint: Find two dependent events define the rom variables in the following way. if if 28. Two fair coins, each with faces numbered 1 2, are thrown independently. Let denote the sum of the two numbers obtained, let denote the maximum of the numbers obtained. Find the correlation coefficient between 3. CONDITIONAL DIsTRIBUTIONs AND ExpECTATIONs Definition: Let be rom variables. The conditional probability distribution function of, given that is defined by Suppose are discrete rom variables assuming the values respectively. Then for each such that we define the conditional probability mass function of given that by The conditional expectation of given that is defined by Suppose are jointly continuous with the joint density function Then the conditional probability density function of given that is defined for all values of such that by

13 The conditional expectation of given that is defined for all values of such that by Theorem: (Law of Total Expectation) Let be jointly distributed rom variables. Then PROBLEM EXAMPLE 1 A young man a young lady plan to meet between 5 6 p.m., each agreeing not to wait more than ten minutes for the other. Assume that they arrive independently at rom times between 5 6 p.m. Find the probability that the young man young lady will meet, given that the young man arrives at 5:30 p.m. Let be the man s arrival time (in minutes after 5 p.m.). If the man arrives at a time, there will be meeting iff the lady s arrival time satisfies Let denote the event that the man lady meet. Then, for any between Using the fact that is uniformly distributed between we obtain Consequently, PROBLEM EXAMPLE 2 Suppose the rom variables have the joint density function Find We know that the marginal density function of is given by

14 Hence, if Thus, is independent of the particular value of This is to be expected since are independent. (They are independent since ) Hence, for each This integral is not difficult to evaluate its value is 4. ExERCIsEs visible invisible 1. Let the joint distribution function of be uniform on the unit disk. Calculate the conditional density function of, given that if if 2. Let be uniformly distributed on the triangle Calculate if 3. Let the joint density function of be Calculate if 4. A fair die is thrown times independently. Let be the number of six be the number of even. Show

15 that 5. Let be jointly normally distributed with joint probability density function Calculate 6. Let be jointly normally distributed with parameters i. ii. Find (i) 0.28, (ii) The number of devices received during a day by a repair shop represents the rom variable distributed according to a Poisson law with the expectation 4. The probability that a received device will require extensive repair is Find the expectation of the number of devices received during a day which require an extensive repair. 8. Given jointly distributed rom variables, prove that, for any almost all if only if are independent. 9. Let a rom sample of size be drawn with out replacement from an urn containing balls. Suppose that the number of white balls in the urn is a rom variable. Let be the number of white balls contained in the sample. Calculate the conditional distribution of given that 10. Let be a rom variable, let be a fixed number. Define the rom variable by the event by Evaluate in terms of the distribution function of Explain the difference in meaning between these concepts.

16 11. If are independent Poisson rom variables, show that the conditional distribution of given is binomial. 12. Consider electronic tubes of a certain type whose lifetimes are normally distributed with parameters Let a rom sample of four tubes be put into a box. Choose a tube at rom from the box. What is the probability that the tube selected will have a lifetime greater than 180 hours? Find the conditional expectation of, given if are jointly continuous rom variables with a joint probability density function vanishing except for in the case in which given by a. b. c. (a) (b) (c) 5. TRANsfORm METHODs Definition: Let be a discrete rom variable assuming only nonnegative integer values let Then the function defined by is called the generating function of

17 Theorem: (Properties of the Generating Function) Let be discrete rom variables assuming only nonnegative integer values. Then the following hold. (a) converges at least for (b) have the same distribution iff (uniqueness). (c) (d) (e) if are independent. Definition: The characteristic function of a rom variable is defined by for all real Theorem: (Properties of the Characteristic Function) Let be rom variables. Then the following hold. (a) iff (uniqueness). (b) (c) for all if are independent. (d) if exists. Theorem: If the characteristic function is absolutely integrable then the rom variable has probability density function given by Definition: The probability distribution function of a rom variable which arises as a function of another

18 rom variable is given by Theorem: If is differentiable for all either for all or for all if is a continuous rom variable, then is continuous rom variable with probability density function given by if otherwise. Definition: The probability distribution function of a rom variable which arises as a function of another rom variables with joint probability is given by Definition: The joint probability function of rom variables which arise as functions of continuous rom variables such that is given by Now we consider only the case in which the functions all points are such that the Jacobian matrix have continuous first partial derivatives at for all points. If are jointly continuous rom variables, whose joint probability density function is the point is exactly one solution of the system of equations

19 then the rom variables defined by are jointly continuous with a joint probability density function given by PROBLEM EXAMPLE 1 Consider a rom variable with a probability density function, for some positive constant which is called Laplace s distribution. Find the characteristic function of We note that since is an even function of we may write PROBLEM EXAMPLE 2 Determine the probability generating function, expectation variance of a geometric rom variable The probability mass function is function of is Thus the probability generating Then Hence PROBLEM EXAMPLE 3 Let in which the amplitude is a known phase is a rom variable uniformly distributed on the interval to Calculate The probability distribution function for is given by

20 Consequently, the probability density function is given by PROBLEM EXAMPLE 4 Let be jointly continuous rom variables. Let For any real numbers show that Let The equations clearly have as their solution The Jacobian is given by 6. ExERCIsEs visible invisible 1. Determine the generating function, the expectation the variance of the binomial, negative binomial Poisson distribution. 2. Determine the characteristic functions of the following distributions: exponential distribution with parameter Cauchy with parameters 3. Find the probability laws corresponding to the following characteristic functions: i. ii.

21 iii. iv. (i) (ii) Cauchy: (iii) Poisson: (iv) if 4. Let be the sum of independent rom variables. Prove that if, for, is i. then is ii. then is iii. then is iv. Cauchy distributed with parameters then is Cauchy distributed with parameters 5. Find the mean the variance of the number of throws required of a fair die until i. a six is obtained, ii. 10 six is obtained. (i) 6, (ii) Let be a sequence of rom variables with binomial distribution so that in which is constant. Prove that 7. Calculate the expected value variance of the number of throws of a normal unbiased die required to obtain the sequence (not necessarily consecutively). 36, Let be rom variable with distribution function density function Determine the distribution density function of where are given real numbers such that

22 9. The rom variable has strictly increasing distribution function Prove that the rom variable is uniformly distributed on the interval to 10. Let be independent rom variables with commom exponential distribution. Find the densities of (i) (ii) (iii) (iv) (v) the smaller of (vi) the larger of Do the same problems if are (i) for all (ii) 11. Find the distribution functions of if the variables have a common exponential distribution. (i) (ii) 12. Find the density function of the rom variables the rom variable being uniformly distributed in the interval

23 13. Let be independent rom variables with common distribution Find i. ii. iii. iv. v. (i) 0.125, 0.875, (ii) (iii) 0.393, (iv) (v) 14. Find the probability density function of in which is The rom variable is said to have a lognormal distribution with parameters 15. Let be uniformly distributed on (a) the interval to (b) the interval to Find sketch the probability density function of the functions given. (i) (ii) (iii) (iv) (i) (a) (b) (ii) (a) (iii) (a) (b) (iv) 16. The magnitude of the velocity of a molecule with mass in a gas at absolute temperature is a rom variable, which, according to the kinetic theory of gas, possesses the Maxwell distribution with parameter in which is Boltzmann s constant Find the probability density function of the kinetic energy pf a molecule. Describe in words the probability

24 law of 17. Let be independent rom variables, each exponentially distributed with parameter Show that the rom variables are independent. 18. Let be independent rom variables, each normally distributed with parameters Show that the rom variables are independent. 19. Let be independent rom variables, each uniformly distributed over the interval to Find the marginal joint probability density functions of the rom variables in which See the answer to Exercise Let be independent rom variables, each exponentially distributed with parameter Find the joint probability density function of i. ii. (i) if (ii) if 21. Let be Find sketch the probability density functions of the functions given. (i) (ii) (iii) (iv) (i) if (ii) if (iv) if 22. Let are independent rom variables, each Calculate the density function of i. ii.

25 iii. (i) (ii) (iii) Student s (degree ), 23. Suppose that the load on an airplane wing is a rom variable obeying a normal probability law with mean variance whereas the load that the wing can withst is a rom variable obeying a normal probability law with mean variance Assuming that are independent, find the probability that (that the load encountered by the wing is less than the load the wing can withst) Let be independent rom variables, Describe the probability distribution of Using the result for forming a rom sample of a rom variable with a degrees of freedom. distribution with if (i) if if (ii) if if 25. The rom variable represents the amplitude of a sine wave; represents the amplitude a cosine wave. Both are independently uniformly distributed on the interval to i. Let the rom variable represent the amplitude of their resultant, that is, Find sketch the probability density function of ii. Let the rom variable represent the phase angle of the resultant, that is, Find sketch the probability density function of

26 if 26. Consider jointly distributed rom variables with a joint probability density function Find the probability density function of the sum Digitális Egyetem, Copyright Fegyverneki Sándor, 2011

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