# am_lect_21 - Method of Unrolling Consider the following...

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Method of Unrolling Consider the following inductively defined function: T (1) = 1 n 2, T ( n ) = T ( n 1) + 3 n Claim: T ( n ) is O ( n 2 ) roof: Suppose very large Then Proof: n is very large. Then T ( n )= T ( n 1) + 3 n (by definition) = T ( n 2) + 3(( n 1) + n ) = T ( n 3) + 3(( n 2) + ( n 1) + n ) = ) + 3(2 + 3 + … + ( ) + ( ) + T (1) 3(2 3 ( n 2) ( n 1) n ) = hich is ) 1 ( 2 3 1 2 ) 1 ( 3 1 1 3 1 3 1 n n n n i i n n which is O ( n 2 ) 1 2 i i

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Method of Unrolling: Another example Consider the following inductively defined function: M (0) = p n 2, M ( n ) = (1.05) M ( n 1) Claim: T ( n ) is O (1.05 n ) roof: Suppose very large Then Proof: n is very large. Then M ( n ) = (1.05) M ( n 1) (by definition) = (1.05) 2 M ( n 2) = (1.05) 3 M ( n 3) = (1.05) n ) M (0) = (1.05) n p hich is 05 which is O (1.05 n )
Sample proofs with Big O We have just shown that M ( n ) is O (1.05 n ) Claim 1: M ( n ) is O (2 n ) Proof 1: Since
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am_lect_21 - Method of Unrolling Consider the following...

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