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Assessment 2 knuth book

# Set cl contfl c2 conte2 c gcdcl c2 2 set f fxcl f2

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Set Cl = cont(Fl'), c2 = cont(E2'), c = gcd(Cl, c2). (2) Set F~ = Fx'/Cl, F2 = F2'/c2. (3) Set fl = lc (F1), f2 = lc (F~), 0 = gcd (.,q, f2). (4) Seth = 0, e = min(a(F~), O(F2)). (5) Set fi = 20 max I~1, where ¢ ranges over the coefficients of F1 and F2. Usually, it will be true that fi > p, but exceptions are possible as discussed in Section 5.2. (6) Let p be a new odd prime not dividing fl or fi. (7) Set ~ = g mod p, F1 = 0F1 mod p, F2 = ~F2 mod p. (S) Invoke Algorithm P (Section 4.5) to compute (~ = 0'gcd(F1, F2),/ql = F1/G, and/t: = F2/G, all in Zv[xl, • • • , x~]. These relations imply that lc (G) = 0, and ~(0) > d. (9) If it(G) = 0, set G = 1, H~ = F1,H2 = F2, and skip to Step (15). If O(G) > e, go back to step 6. If i)(G) < e, set n = 0, e = i)(G). (10) Setn = n + 1. (11) Ifn = 1, setq = p,G* = G,H,* = /ql,H2* =/72.Otherwise, updatethe quadruple (q, G*, Hi*, H2*) to include (p, G, H1,/q2) by using the Chinese remainder algorithm (Section 4.8) with moduli ~nl = q and ~ = p to extend (37) (coefficient by coefficient), and then replacing q by pq to extend (36). (12) If q < fi, go back to Step (6). Otherwise, we now know that (40) holds unless e > d or q < g. To exclude these unlikely possibilities, it suffices to prove the relations G'Hi* = F~ and G'H2* = fi2, which hold modulo q by (31), (33), (36), and (37). (13) Choose g* such that ~*/2 is an integer bound on the magnitudes of the coefficients of G*H~* G'H2*. If q < u*, go back to Step (6). Otherwise, we have q] (G'H1* - fl) and q > max (u*, ~) _> max I~ [, where ~, ranges over the coef- ficients of (G'Hi* - F1), and therefore G'Hi* = fix. Similarly, G'H2* = fi2, and therefore (40) is established. (14) Set G = pp(G*), g = lc(G), H~ = Hl*/g, H~ = H2*/g. (15) Set G' = cG, Hi' = (Cl/C)H1, H~' = (c2/c)H:, and return. 4.4 UNLUCKY PR~IES. We shall call the prime p chosen in Step (6) of Algorithm 5I lucky if e = d, and unlucky otherwise. In executing Algorithm 5I, the first lucky prime causes us to discard the informa- tion from any previous unlucky primes [by setting n = 0 in the third part of Step (9)], and to set e = d. Any subsequent unlucky primes are rejected in the second part of Step (9). In Step (13), G*, H~*, and H:* are rejected if no lucky primes have yet been encountered, or if q is still less than u. In Theorem 1, we shall prove that all of the unlucky primes are divisors of an integer a, which depends only on F1 and F:. Using this result, Theorem 2 bounds the number of unlucky primes which might occur, thereby establishing the fact that the algorithm terminates. Finally, Theorem 3 shows that the probability of p being un- lucky is at most v/p. In practice, this probability is always exceedingly small, since p is chosen in the Journal of the Association for Computing Machinery, Vol. 18, No. 4, October 1971

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492 w.s. BROWN interval a = ½(/~ + 1) < p _< ~, (43) where ¢~ is the largest integer that fits in a single machine word. Thus, if F~ and F: are relatively prime, we can expect to prove it with only a single prime. Otherwise, the expected number of primes to determine the GCD and the eofaetors is g = log, t~ , where ~ is the final value of ~* in Step (13), and it can be shown [see (94)] that g rarely exceeds 4l + 2 where I is the length (to the base o~) of the longest coefficient in F~ or F~.
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Set Cl contFl c2 contE2 c gcdCl c2 2 Set F FxCl F2 F2c2 3...

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