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# 02-asymp - Asymptotic Notation Review of Functions...

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March 26, 2014 Comp 122, Spring 2004 Asymptotic Notation, Review of Functions & Summations Asymptotic Notation, Review of Functions & Summations

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Comp 122 ymp - 2 Asymptotic Complexity Running time of an algorithm as a function of input size n for large n . Expressed using only the highest-order term in the expression for the exact running time. Instead of exact running time, say Θ ( n 2 ). Describes behavior of function in the limit. Written using Asymptotic Notation .
Comp 122 ymp - 3 Asymptotic Notation Θ , O , , o , ϖ Defined for functions over the natural numbers. Ex: f ( n ) = Θ ( n 2 ). Describes how f ( n ) grows in comparison to n 2 . Define a set of functions; in practice used to compare two function sizes. The notations describe different rate-of-growth relations between the defining function and the defined set of functions.

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Comp 122 ymp - 4 Θ -notation Θ ( g ( n )) = { f ( n ) : 5 positive constants c 1 , c 2 , and n 0, such that 2200 n n 0 , we have 0 c 1 g ( n ) f ( n ) c 2 g ( n ) } For function g ( n ), we define Θ ( g ( n )), big-Theta of n , as the set: g ( n ) is an asymptotically tight bound for f ( n ). Intuitively : Set of all functions that have the same rate of growth as g ( n ).
Comp 122 ymp - 5 Θ -notation Θ ( g ( n )) = { f ( n ) : 5 positive constants c 1 , c 2 , and n 0, such that 2200 n n 0 , we have 0 c 1 g ( n ) f ( n ) c 2 g ( n ) } For function g ( n ), we define Θ ( g ( n )), big-Theta of n , as the set: Technically, f ( n ) Θ ( g ( n )). Older usage, f ( n ) = Θ ( g ( n )). I’ll accept either… f ( n ) and g ( n ) are nonnegative, for large n .

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Comp 122 ymp - 6 Example 10 n 2 - 3 n = Θ ( n 2 ) What constants for n 0 , c 1 , and c 2 will work? Make c 1 a little smaller than the leading coefficient, and c 2 a little bigger. To compare orders of growth, look at the leading term. Exercise: Prove that n 2 /2-3 n = Θ ( n 2 ) Θ ( g ( n )) = { f ( n ) : 5 positive constants c 1 , c 2 , and n 0 , such that 2200 n n 0 , 0 c 1 g ( n ) f ( n ) c 2 g ( n ) }
Comp 122 ymp - 7 Example Is 3 n 3 Θ ( n 4 ) ?? How about 2 2 n Θ (2 n )?? Θ ( g ( n )) = { f ( n ) : 5 positive constants c 1 , c 2 , and n 0 , such that 2200 n n 0 , 0 c 1 g ( n ) f ( n ) c 2 g ( n ) }

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Comp 122 ymp - 8 O -notation O ( g ( n )) = { f ( n ) : 5 positive constants c and n 0, such that 2200 n n 0 , we have 0 f ( n ) c g ( n ) } For function g ( n ), we define O ( g ( n )), big-O of n , as the set: g ( n ) is an asymptotic upper bound for f ( n ). Intuitively : Set of all functions whose rate of growth is the same as or lower than that of g ( n ).
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