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Unformatted text preview: Random Number Generation 1 Introduction Anyone who considers arithmetical methods of producing random digits is, of course, in a state of sin. This famous statement concerning the use of sequences of numbers generated using recursive formulas as random sequences is attributed to John von Neumann (1951). To generate a random number u from the Uniform (0,1) distribution, one must randomly select an integer x (0 , m ) and set u = x m . Machinegenerated sequences of real numbers depend on recursions of the form: x j = f ( x j 1 , x j 2 , . . . , x j k ) , or more expeditiously, x j = f ( x j 1 ) . Such sequences are obviously deterministic . We want to be able to generate sequences whose properties closely resemble those of random sequences in important ways. In addition, since such sequences are also necessarily periodic , we want such generators to have long periods, as well. 2 Linear Congruential Generators Linear Congruential Generators are defined by the recursion x i +1 ( a x i + c ) mod m where x , x 1 , . . . is a sequence of integers and depends upon x : a seed a : a multiplier c : a shift m : a modulus all of which are also integers defines an equivalence relation. Two numbers a and b are said to be congruent modulo m or a b mod m where m is an integer, if their difference is exactly divisible by m . If 0 a < m and a b mod m , then a is said to be a residue of b modulo m . a can be easily calculated using a = b b/m m , where the floor function x computes the greatest integer less than x . The pseudorandom sequence { u i } is obtained by setting u i = x i /m for i = 1, 2, . . . . If c = 0, the above defines a multiplicative congruential generator (MCG). Some theoretical discussions on MCGs are given in Ripley(1987), Gentle(1998, 2003) and Fishman(2006). A typical choice for m on binary computers is 2 k where k is the typical machine length 1 for integer type storage. In early generators this was thought to be a good choice because then the modulo reduction can be achieved simply via standard fixedpoint overflow in the integer multiplication operation ( a x i ). Recall that if fixedpoint overflow is not trapped, the result of an integer arithmetic multiplication is modulo reduced by 2...
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This note was uploaded on 02/01/2012 for the course STAT 330B taught by Professor Zhou during the Spring '11 term at Iowa State.
 Spring '11
 Zhou

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