# 280wk2_x4 - Matrix Multiplication Given two vectors a =...

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Matrix Multiplication Given two vectors ~a = [ a 1 , . . . , a k ] and ~ b = [ b 1 , . . . , b k ], their inner product (or dot product ) is ~a · ~ b = k X i =1 a i b i [1 , 2 , 3] · [ - 2 , 4 , 6] = (1 ×- 2) + (2 × 4) + (3 × 6) = 24. We can multiply an n × m matrix A = [ a ij ] by an m × k matrix B = [ b ij ], to get an n × k matrix C = [ c ij ]: c ij = m r =1 a ir b rj this is the inner product of the i th row of A with the j th column of B 1 2 3 1 5 7 4 × 3 7 4 2 - 1 - 2 = 17 18 39 41 17 = (2 × 3) + (3 × 4) + (1 × - 1) = (2 , 3 , 1) · (3 , 4 , - 1) 18 = (2 × 7) + (3 × 2) + (1 × - 2) = (2 , 3 , 1) · (7 , 2 , - 2) 39 = (5 × 3) + (7 × 4) + (4 × - 1) = (5 , 7 , 4) · (3 , 4 , - 1) 41 = (5 × 7) + (7 × 2) + (4 × - 2) = (5 , 7 , 4) · (7 , 2 , - 2) 2 Why is multiplication deFned in this strange way? Because it’s useful! Suppose z 1 = 2 y 1 + 3 y 2 + y 3 y 1 = 3 x 1 + 7 x 2 z 2 = 5 y 1 + 7 y 2 + 4 y 3 y 2 = 4 x 1 + 2 x 2 y 3 = - x 1 - 2 x 2 Thus, z 1 z 2 = 2 3 1 5 7 4 · y 1 y 2 y 3 and y 1 y 2 y 3 = 3 7 4 2 - 1 - 2 · x 1 x 2 . Suppose we want to express the z ’s in terms of the x ’s: z 1 = 2 y 1 + 3 y 2 + y 3 = 2(3 x 1 + 7 x 2 ) + 3(4 x 1 + 2 x 2 ) + ( - x 1 - 2 x 2 ) = (2 × 3 + 3 × 4 + ( - 1)) x 1 + (2 × 7 + 3 × 2 + ( - 2)) x 2 = 17 x 1 + 18 x 2 Similarly, z 2 = 39 x 1 + 41 x 2 . z 1 z 2 = 2 3 1 5 7 4 · 3 7 4 2 - 1 - 2 · x 1 x 2 . 3 Algorithms An algorithm is a recipe for solving a problem. In the book, a particular language is used for describing algorithms. You need to learn the language well enough to read the examples You need to learn to express your solution to a prob- lem algorithmically and unambiguously YOU DO NOT NEED TO LEARN IN DETAIL ALL THE IDIOSYNCRACIES O± THE PARTICULAR LANGUAGE USED IN THE BOOK. You will not be tested on it, nor will most of the questions in homework use it I suggest you skim Chapter 1; I won’t cover it 4

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Methods of Proof One way of proving things is by induction. That’s coming next. What if you can’t use induction? Typically you’re trying to prove a statement like “Given X , prove (or show that) Y ”. This means you have to prove X Y In the proof, you’re allowed to assume X , and then show that Y is true, using X . A special case: if there is no
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## This note was uploaded on 05/21/2009 for the course CS 2800 taught by Professor Selman during the Spring '07 term at Cornell University (Engineering School).

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280wk2_x4 - Matrix Multiplication Given two vectors a =...

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