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# For(v one proves the statement for the t i by

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Unformatted text preview: For (v), one proves the statement for the t i by induction, but with the stronger hypothesis that t i t i +1 ≤ 0 (i.e., the sign alternates) and | t i | ≤ | t i +1 | for 0 ≤ i ≤ ‘ (exercise). One argues similarly for the statement for the s i . (vi) follows immediately from (iv) and (v). 2 Example 3.2 We continue with Example 3.1. The numbers s i and t i are easily computed from the q i : i 1 2 3 4 r i 100 35 30 5 q i 2 1 6 s i 1 1-1 7 t i 1-2 3-20 2 We can easily turn the scheme described in Theorem 3.5 into a simple algorithm, as follows: s ← 1 , t ← s ← , t ← 1 while b 6 = 0 do Compute q,r such that a = bq + r , with 0 ≤ r < b ( s,t,s ,t ) ← ( s ,t ,s- s q,t- t q ) ( a,b ) ← ( b,r ) output a,s,t 16 This algorithm, known as the extended Euclidean algorithm , computes the greatest common divisor d of a and b , together with s and t such that as + bt = d . Theorem 3.6 The extended Euclidean algorithm runs in time O ( L ( a ) L ( b )) . Proof. It suffices to analyze the cost of computing the sequences { s i } and { t i } . Consider first the cost of computing all of the t i , which is O ( τ ), where τ = ∑ ‘ i =1 L ( t i ) L ( q i ). By Theorem 3.5 part (vi), and arguing as in the proof of Theorem 3.4, we have τ = L ( q 1 ) + ‘ X i =2 L ( t i ) L ( q i ) ≤ L ( q 1 ) + L ( a )( ‘- 1 + log 2 ( ‘ Y i =2 q i )) = O ( L ( a ) L ( b )) , using the fact that Q ‘ i =2 q i ≤ b . An analogous argument shows that one can compute all of the s i also in time O ( L ( a ) L ( b )), and in fact, in time O ( L ( b ) 2 ). 2 We should point out that the Euclidean algorithm is not the fastest known algorithm for com- puting greatest common divisors. The asymptotically fastest known algorithm for computing the greatest common divisor of two numbers of bit length at most k runs in time O ( k (log k ) 2 log log k ). One can also compute the corresponding values s and t within this time bound as well. Fast algo- rithms for greatest common divisors are not of much practical value, unless the integers involved are very large — at least several tens of thousands of bits in length. 3.4 Computing in Z n Let n > 1. For computational purposes, we may represent elements of Z n as elements of the set { ,...,n- 1 } . Addition and subtraction in Z n can be performed in time O ( L ( n )). Multiplication can be performed in time O ( L ( n ) 2 ) with an ordinary integer multiplication, followed by a division with remainder. Given a ∈ { ,...,n- 1 } , we can determine if [ a mod n ] has a multiplicative inverse in Z n , and if so, determine this inverse, in time O ( L ( n ) 2 ) by applying the extended Euclidean algorithm. More precisely, we run the extended Euclidean algorithm to determine integers d , s , and t , such that d = gcd( n,a ) and ns + at = d . If d 6 = 1, then [ a mod n ] is not invertible; otherwise, [ a mod n ] is invertible, and [ t mod n ] is its inverse. In the latter case, by part (vi) of Theorem 3.5, we know that | t | ≤...
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