C__Documents_and_Settings_raghu_Desktop_CS_2800_hw5_hw5-soln

# C__Documents_and_Settings_raghu_Desktop_CS_2800_hw5_hw5-soln...

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Unformatted text preview: CS 2800 – Fall 2008 Homework #5 Solution Section 3.4 Problem 6 [4pts] By definition of the | operator, we have c = as and d = bt , for non-zero integers s, t . Multiplying these equations together, we have cd = ab ( st ), or ab | cd as desired. Problem 16 [4pts] Note that by definition, a mod b is a positive integer. This is of particular importance when computing the modulo of a negative number — it means that the quotient will often be one greater in magnitude than if we were simply dividing the number. (a)- 17 = 2 · (- 9) + 1. Hence,- 17 mod 2 = 1. (b) 144 = 7 · 20 + 4. Hence, 144 mod 7 = 4. (c)- 101 = 13 · (- 8) + 3. Hence,- 101 mod 13 = 3. (d) 199 = 19 · 10 + 9. Hence, 199 mod 19 = 9. Problem 22 [4pts] From a ≡ b (mod m ), we have a = mt + b for some integer t ≥ 0. Multiplying throughout by c (since c > 0), we have ac = mtc + bc = mc ( t ) + bc i.e. ac ≡ bc (mod mc ) as desired. Problem 24 [5pts] Since n is a positive odd integer, it can be written as 2 k +1 where k is a non-negative integer. Then, we have n 2 = (2 k + 1) 2 = 4 k 2 + 4 k + 1 = 4 k ( k + 1) + 1. Since either k or k + 1 is even, 4 k ( k + 1) is divisible by 8 and therefore, 4 k ( k + 1) + 1 = n 2 ≡ 1 (mod 8). Section 3.5 Problem 14 [6pts] (a) 6 is perfect since 6 = 1 + 2 + 3. 28 is perfect since 28 = 1 + 2 + 4 + 7 + 14. 1 (b) Firstly, we observe that the factors of 2 p- 1 (2 p- 1) are of the form 2 k (2 p- 1) a where a ∈ { , 1 } and 0 ≤ k ≤ p- 1. Based on the value of a , we can view these factors as forming two different geometric series: • a = 0 : In this case, we get the factors 2 , 2 1 , . . . , 2 p- 1 . The sum of these factors is 2 p- 1....
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## This note was uploaded on 12/04/2008 for the course CS 2800 taught by Professor Selman during the Fall '07 term at Cornell.

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C__Documents_and_Settings_raghu_Desktop_CS_2800_hw5_hw5-soln...

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