448 ARITHMETIC 4.6.3 Theorem A. If the addition (Web hosting control panel)
Monday, April 30th, 2007448 ARITHMETIC 4.6.3 Theorem A. If the addition chain (11) includes d doublings and f = r -d nondoublings, then n 2 2*- Ff+s. (14) Proof. By induction on r = d + f, we see that (14) is certainly true when r = 1. When r > 1, there are three cases: If step r is a doubling, then $n = a,-1 5 2d-2F f+s; hence (14) follows. If steps r and T -1 are both nondoublings, then a,-1 5 2*-lFf+2 and arP2 5 2d-1Ff+l; hence n = a,. 2 a,.-1 + a7-s 2 2*- (Ff-t2 + Fffl) = 2d-1Ff+3 by the definition of the Fibonacci sequence. Finally, if step r is a nondoubling but step r -1 is a doubling, then arP2 2 2*- Ff+z and n = a,. 5 a,_l + a7-2 = 3a7-2. Now 2Ff+s -3Ff+2 = Fftl -Ff 2 0; hence n 5 2d-1Ff+3 in all cases. 1 The method of proof we have used shows that inequality (14) is best possible under the stated assumptions; the addition chain 1,2,. . . ,2*–1, 2*–lF3, 2*-lF 4,*.., 2*–lFf+s (15) has d doublings and f nondoublings. Corollary. If the addition chain (11) includes f nondoublings and s small steps, then s 5 f 2 3.271s. (16) Proof. Obviously s 6 f. We have ZXcn) 2 n 5 2d-1Ff+3 5 2*4f = 2X(n)+s(+/2)f, since d + f = A(n) + s, and since Ff+s < 24f when f > 0. Hence 0 5 s In 2 + f ln(#/2), and (16) follows from the fact that In 2/ ln(2/@) M 3.2706. 1 Values of l(n) for special 12. It is easy to show by induction that ai 5 2i, and therefore lgn 5 r in any addition chain (11). Hence l(n) 2 knl. (17) This lower bound, together with the upper bound (10) given by the binary method, gives us the values 1(2A) = A; (18) 1(2A+2B)=A+1, ifA> B. (19) In other words, the binary method is optimum when u(n) 2 2. With some further calculation we can extend these formulas to the case v(n) = 3:
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