DivideNConquer-II

# DivideNConquer-II - Divide and Conquer(Part II...

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Divide and Conquer (Part II) ultiplication of two numbers Multiplication of two numbers Let U = ( u 2 n -1 u 2 n -2 u 1 u 0 ) 2 and V = ( v 2 n -1 v 2 n -2 v 1 v 0 ) 2 , and our goal is to find U times V . Ordinary multiplication requires execution time α ( n 2 ). Alternatively, let U = 2 n U 1 + U 0 and V = 2 n V 1 + V 0 where U 1 = ( u 2 n -1 u 2 n -2 u n ) 2 and U 0 = ( u n -1 u n -2 u 0 ) 2 Likewise, V 1 = ( v 2 n -1 v 2 n -2 v n ) 2 and V 0 = ( v n -1 v n -2 v 0 ) 2 U X V = 2 2n U 1 V 1 + 2 n ( U 1 V 0 + U 0 V 1 ) + U 0 V 0 . Apparently there is no saving, this multiplication also requires execution time proportional to n 2. 1/21/2010

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Divide and Conquer (Part II) ultiplication: Divide and Conquer Multiplication: Divide and Conquer U X V = (2 2n + 2 n ) U 1 V 1 + 2 n ( U 1 –U 0 )( V 0 –V 1 ) + (2 n + 1) U 0 V 0 . which requires only three multiplications and some extra addition. If T ( n ) denotes time to multiply two binary integers of size hth n each, then T (2 n ) = 3 T( n ) + c n , and T (1) =1, where cn denotes the cost associated with additions and shifting of binary integers etc. . 1/21/2010
Divide and Conquer (Part II) ultiplication: Divide and Conquer Multiplication: Divide and Conquer Solution of this recurrence equation is (by Master Theorem) or repeated substitution, for n = 2 m , is T (2 m ) = 3 m T (1) + c (3 m -2 m ) In general, lg n ) + c(3 lg n - lg n T ( n ) 3 T (1) + c(3 2 ) 3c X 3 lg n 3 585 = 3c X n lg 3 3c X 1.585 1/21/2010

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Divide and Conquer (Part II) atrix ultiplication Matrix Multiplication Let C = AB = ( c ij ) be the product of two n x n matrices A and B , where a 11 a 12 a 1 n b b 12 b 1 n A = a 21 a 22 a 2 n and B = b 21 b 22 b 2 n a n 1 a n 2 a nn b n 1 b n 2 b nn • We calculate n 2 terms, c ij for i =1,2,…, n , , j =1,2,…, n . ach quires alar multiplications Each c ij requires n scalar multiplications. • Therefore, total complexity of this matrix multiplication is of the order n 3 . 1/21/2010
Divide and Conquer (Part II) Suppose n = 2 k for some integer value of k . Let r sa be g C = = A X B = X t uc df h Where r,s,t,u are all n/2 X n/2 matrices • In this way of obtaining C , by first partitioning A and B in submatrices, we calculate 4 matrices r , s , t , and u .

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DivideNConquer-II - Divide and Conquer(Part II...

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