# lec2 - n implying f n is O g n constant f n is Θ g n...

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Rule of the game Our Question: can we do “better”? What is “better”? Less running time Less space usage How to measure? Asymptotic notation In Which Environment? RAM model: our virtual computer * Random memory access 1

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* All memory accesses are the same, done in constant time * This is not a real computer How we program? Pseudocode No worry about syntax details Looks like Java code Implementable by programmers (like you) 2
Asymptotic Notations and Deﬁnitions Assume f ( n ) and g ( n ) are functions from the non-negative real numbers to the non-negative real numbers. We deﬁne: Notation Deﬁnition f ( n ) is c > 0 and n 0 > 0, O ( g ( n )) s.t. ( n n 0 )( f ( n ) cg ( n )) f ( n ) is c > 0 and n 0 > 0, Ω( g ( n )) s.t. ( n n 0 )( f ( n ) cg ( n )) f ( n ) is f ( n ) is O ( g ( n )) and = Θ( g ( n )) f ( n ) is Ω( g ( n )) f ( n ) is c > 0 , n 0 > 0 < o ( g ( n )) s.t. ( n n 0 )( f ( n ) < cg ( n )) f ( n ) is c > 0 , n 0 > 0 > ω ( g ( n )) s.t. ( n n 0 )( f ( n ) > cg ( n )) Some examples 3

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Prove Using Limits We compute lim n →∞ f ( n ) g ( n ) Value Asymptotic bound 0 f ( n ) is o ( g
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Unformatted text preview: ( n )), implying f ( n ) is O ( g ( n )) constant f ( n ) is Θ( g ( n ), implying f ( n ) is O ( g ( n )) and Ω( g ( n )) ∞ f ( n ) is ω ( g ( n )), implying f ( n ) is Ω( g ( n )) Some examples 4 Useful Math Formulae • Logorithmic – x = log a b ⇔ a x = b – log a b = log c a log c b – a log b c = c log b a – log a +log b = log ( ab );log a-log b = log a b – log( a b ) = b log a • Series and Summation – ∑ n i =1 i = n ( n +1) 2 – ∑ n-1 i =0 q i = 1-q n 1-q (Geometric series) – ∑ n i =1 1 i is O (log n ) (Harmonic series) • L’Hˆopital rule (our commonly used version) If lim n →∞ f ( n ) = lim n →∞ g ( n ) = ∞ Then lim n →∞ f ( n ) g ( n ) = lim n →∞ f ( n ) g ( n ) 5...
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## This note was uploaded on 07/17/2010 for the course CS 240 taught by Professor Ortiz during the Spring '09 term at Waterloo.

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lec2 - n implying f n is O g n constant f n is Θ g n...

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