Generalization of Vieta’s Formula AlJalila Al-Abri Department of Mathematics and Statistics Sultan Qaboos University P.O.Box 36, Al-Khoud 123 Muscat asalabri@hotmail.com 1 Introduction In 1579 Francois Vieta (1540-1603) derived the following formula q p p p p p 2 + 2+ 2 2 2 2+ 2 = 2 2 2 (1) to approximate the value of . He obtained this formula, which subsequently became known as Vieta’s formula using a geometric approach [3]. As it is often the case, things can be achieved in many di¤erent ways in mathematics. Vieta’s formula can be derived from the identity x sin x Y = cos k x 2 k=1 1 (2) by setting x = 2 , and identity (2) can be obtained by using the elementary trigonometric identity sin 2x = 2 sin x cos x and elementary calculus. In his beautiful monograph [1] Mark Kac (1914 - 1984) began with a proof of Vieta’s formula using the Rademacher functions and their independence property, and in the …rst chapter, a generalization of Vieta’s formula was suggested as an exercise. In this paper we provide a proof following Kac’s idea of using the independence property of the Rademacher functions. To the best of our knowledge, this generalization has only been achieved by Kent E. Morrison using the Fourier transform and delta distributions (see [2]). In the following section we introduce some facts about the expansion of numbers in the closed interval [0; 1] to any base g 2: In section 3 we brie‡y present the Rademacher functions and their independence property. In section 4 we give the generalization and its proof using the Rademacher functions and their independence property. In section 5 we use the general formulas to approximate the value of and give a comparison of di¤erent formulas: 1 2 Preliminaries Real numbers in the interval [0; 1] can be represented using the binary expansion. In fact, each real number 0 t 1 can be written as "1 "2 "3 t= + 2+ 3+ 2 2 2 where each value of "i is either 0 or 1. For example 3 1 0 0 1 1 0 0 1 1 = + 2+ 3+ 4+ 5+ 6+ 7+ 8+ 9+ : 5 2 2 2 2 2 2 2 2 2 To ensure uniqueness we also impose a terminating expansion in which all the digits from a certain point on are all equal to 0. For example, we write 3 1 1 0 0 0 = + 2+ 3+ 4+ 5+ 4 2 2 2 2 2 instead of 3 1 0 1 1 1 = + 2+ 3+ 4+ 5+ 4 2 2 2 2 2 The digits "i depend on t, so they are functions of t. Hence we write "1 (t) "2 (t) "3 (t) + 2 + 3 + 2 2 2 This idea can of course be generalized to any integer base g we can write ! 1 (t) ! 2 (t) ! 3 (t) + + + t= g g2 g3 where t 2 [0; 1] and ! k 2 f0; 1; ; g 1g. t= 3 2, so in general (3) Rademacher functions In this section we introduce the functions rk (t) which are de…ned in terms of the binary digits "k (t) as follows rk (t) = 1 2"k (t) k = 1; 2; 3; . (4) These functions were …rst introduced by Hans Rademacher (1892 - 1969) as a system of orthogonal functions. He introduced them in a paper which was published in 1922 (more information can be found in [4] ), and these functions are now known as the Rademacher functions. For illustrative purposes, the graphs of the …rst four functions r1 (t) ; r2 (t) ; r3 (t) ; r4 (t) are given below. r1 (t) r2 (t) 2 r3 (t) r4 (t) Notice from the graphs above that rn (t), takes the values +1 or 1 alternatively over the intervals ( 2sn ; s+1 ); s = 0; ; 2n 1; and so the length of each small 2n interval is 21n : Also we deduce that the function rn (t) has 2n intervals, half of them have the value +1 and the other half have the value 1. The Rademacher functions satisfy the independence property fr1 (t) = 1 ; r2 (t) = 2; ng ; rn (t) = = n Y frk (t) = k=1 kg (5) where stands for the measure (length) of the set de…ned inside the braces and n ; n is a sequence of +1 and 1: Thus the product in (5) is 12 . 1; 3.1 Generalization of the Rademacher functions As we can have an expansion to any base for the numbers in the interval [0; 1], we can also de…ne the Rademacher functions as rk (t) = 1 2! k (t) g 1 (6) k = 1; 2; 3; where ! k 2 f0; 1; ; g 1g and g is a positive integer greater than 1: Moreover, the functions de…ned above can also be shown to be independent. Therefore, we have fr1 (t) = where 1; ; 1 ; r2 (t) = n 2 f1; gg 2; 3 g 5 ; ; 1 g 1 ng = ; rn (t) = ; g11; g even number and 1 ; ; n 2 f1; gg when g is an odd number. 1 ; 1 3 g 5 ; ; 1 g 1 3 ; n Y k=1 frk (t) = (g 5) ; g 1 ; g 2 1 ; 0; g (g 3) ; g 1 2 ; 1 ; kg = 1 g n ; 1g when g is an (g 5) ; g 1 (g 3) ; g 1 1g 4 The general formulas The following theorem generalizes formula (2). Theorem 4.1 Let g be a positive integer 2: If g is even, then 0 g 1 1 2 Y X 2@ sin x (2m 1)x A = cos : x g m=1 gk k=1 (7) If g is odd, then 0 0g 1 1 1 2 X sin x 2mx A @2 @ cos = + 1A . k x g g m=1 k=1 1 Y 1 (8) Proof. Equality (8) can be proved by establishing the next …ve identities: 2t X rk (t) = 1 gk k=1 Z 1 Z 1 sin x (g 1)ix( 1g 2t ) 1 dt = e eix(1 2t) dt = x 0 0 0 g 1 1 Z 1 2 X 1 2mx rk (t) exp (g 1)ix k dt = @2 cos + 1A k g g g 0 m=1 " # Z 1 Z 1 n X 1 2t rk (t) e(g 1)ix( g 1 ) dt = exp (g 1)ix lim dt n!1 gk 0 0 k=1 1 1 g Z 1 " exp (g 0 1)ix lim n!1 n X rk (t) gk k=1 # dt = = lim n!1 lim n!1 Z n 1Y 0 k=1 n Z 1 Y k=1 0 (9) (10) (11) (12) exp (g 1)ix rk (t) dt gk exp (g 1)ix rk (t) dt: gk First, equation (9) can be proved as follows. From (3) and (6) we have t = = 1 X ! k (t) k=1 1 X gk (g 1)(1 rk (t)) . 2:g k k=1 Thus, 2t g 1 X 1 = 1 k=1 g k 4 1 X rk (t) k=1 gk . (13) But 1 X 1 gk 1 . g 1 is a geometric series and converges to Hence k=1 2t g 1 = 1 X rk (t) 1 g 1 gk k=1 which proves equation (9). Then equation (10) can be obtained by a simple integration and the identity eix e 2i ix = sin x: Similarly, equation (11) can be derived by a simple integration and noticing that the function rk (t) is integrable over the intervals ( gsk ; s+1 ); s = 0; ; gk 1 gk 5) 3) and has the values 1; gg 13 ; gg 51 ; ; g 2 1 ; 0; g 21 ; ; (g ; (g ; 1 alternatively g 1 g 1 k over these intervals: Therefore we have g possible values and the set of points t satisfying each value has a total length of g1 . Notice Z 1 rk (t) 1)ix k dt = g k g exp (g 0 1 Z 1 gk 1 exp (g 0 1)ix rk (t) dt: gk and Z 1 gk 1 rk (t) dt gk 0 Z 1k Z gk g g (g 1) (g 1) exp ix dt = exp ix dt + k g 1 g gk 0 gk Z 2k Z g k1 g g (g 3) (g 3) + exp ix dt + exp ix dt k g 2 1 gk g k k g g Z 3k Z g k2 g g (g 5) (g 5) + exp ix dt + exp ix dt k g 3 2 g gk k k exp (g 1)ix g g + + Z + Z g 1 2 gk g 3 2 gk g+1 2 gk g 1 2 gk 2(g 1) exp ix dt + gk exp ix 0(g 1) dt: gk 5 Z g+3 2 gk g+1 2 gk exp ix 2(g 1) dt gk Therefore Z 1 rk (t) exp (g 1)ix k dt g 0 !# " ix(g 1) ix(g 1) ix(g 3) ix(g 3) gk gk gk gk 1 e + e + e + e + = gk 1 k ix(g 5) ix(g 5) 2ix 2ix g e gk + e gk + + e gk + e gk + 1 0 g 1 1 2 1@ X 2mx cos + 1A : = 2 k g g m=1 Equation (12) follows from a trivial substitution in equation (9), and …nally, equation (13) is the where we use the independence property of the Rademacher functions. Since the exponential function is continuous, we have " # " # Z 1 Z 1 n n X X rk (t) rk (t) exp (g 1)ix lim dt = lim exp (g 1)ix dt: k k n!1 n!1 g g 0 0 k=1 k=1 " # n X r (t) k Also since exp (g 1)ix is a sequence of integrable functions on [0; 1] gk k=1 and converges uniformly on [0; 1] to the function exp [ix(1 2t)], we can interchange the limit with the integral. Therefore we have " # " # Z 1 Z 1 n n X X rk (t) rk (t) lim exp (g 1)ix dt = lim exp (g 1)ix dt: k k n!1 0 g g 0 n!1 k=1 k=1 Z 1Y n rk (t) = lim exp (g 1)ix k dt: n!1 0 g k=1 Now we show that " Z 1 lim exp (g n!1 0 1)ix n X rk (t) k=1 gk # dt = lim n!1 n Z Y k=1 0 1 exp (g 1)ix rk (t) dt gk by evaluating the integral over all possible values of the function rk (t) as we did to prove equation (11) and using the independence property of the Rademacher functions. Taking 1 ; ; n to be the possible values of the function rk (t) we have " # Z 1 n X rk (t) exp (g 1)ix dt gk 0 k=1 " # n X 1 X rk (t) = n exp ix (g 1) k g g k=1 " # n X X k = exp ix (g 1) k fr1 (t) = 1 ; r2 (t) = 2 ; ; rn (t) = n g g ; ; k=1 n 1 " n # n X X Y k = exp i (g 1) k frk (t) = k g : g ; ; k=1 k=1 1 n 6 Simple calculations give X 1; = = exp ix n n X Y 1; = ; " ; n k 1) k g (g k=1 exp ix(g 1) k=1 n X Y k=1 k n Z 1 Y k=1 n X exp ix(g exp (g 1) 1)ix 0 k gk # n Y k=1 k frk (t) = frk (t) = gk frk (t) = kg kg kg rk (t) dt: gk Now using equation (11) we obtain formula (8). The formula for the even case can be proved using very similar steps as in the odd case. The only di¤erence is that in step (3) we need to consider the values of the function rk (t) in f1; gg 13 ; gg 51 ; ; g11; g 1 ; 1 ; (g 5) ; g 1 (g 3) ; g 1 1g: If we take the value of g from 2 to 10 alternatively in formula (7) and formula (8) we get 1 Y sin x x = cos k x 2 k=1 = = = = = = = = 1 Y 1 k=1 1 Y 3 2 cos 2x +1 3k 1 x 3x cos k + cos k 2 4 4 k=1 1 Y 1 k=1 1 Y 5 2 cos 2x 4x + cos +1 5k 5k 1 x 3x 5x cos k + cos k + cos k 3 6 6 6 k=1 1 Y 1 k=1 1 Y 7 2 cos 2x 4x 6x + cos k + cos k + 1 k 7 7 7 1 x 3x 5x 7x cos k + cos k + cos k + cos k 4 8 8 8 8 k=1 1 Y 1 k=1 1 Y 9 2 cos 2x 4x 6x 8x + cos k + cos k + cos k + 1 k 9 9 9 9 1 x 3x 5x 7x 9x cos k + cos k + cos k + cos k + cos k 5 10 10 10 10 10 k=1 7 We notice that in general the formulas are in…nite products of sum of cosines. The main di¤erence is that in the odd case we have the term "1" which appears due to the value 0 of rk (t) : In the next section we are going to use these formulas to approximate the value of by setting x = 2 : 5 Approximating the value of eral formulas using the gen- The following table is obtained using a computer program written in C++. The program uses formulas (7) and (8) to get approximations of the value of up to 15 decimal places of di¤erent bases. In this table we report only the base value from 2 to 13, but the program can give an approximation of the value of up to 15 decimal places to any base g 2. The …rst row gives the base value to be substituted in the formula. The second row gives the number of iterations needed to compute up to 15 decimal places. The third row gives the total number of cosines needed to compute up to 15 decimal places. The values of the third row can be obtained by multiplying the number of cosines appearing in the formula corresponding to the base value, by the number of iterations needed to compute up to 15 decimal places. For example, when we use g = 4 in the formula (7), the number of cosines are 2; and from the second row, the number of iterations needed to compute up to 15 decimal places is 12: Therefore the number of cosines needed to compute up to 15 decimal places are 2 12 = 24: base value (g) 2 3 4 5 6 7 8 9 10 11 12 13 I(g) 24 15 12 11 10 9 8 8 8 7 7 7 C(g) 24 15 24 22 30 27 32 32 40 35 42 42 From the table we can observe the following: 1. As the base value g get larger, the number of iterations needed to compute up to 15 decimal places decreases. This seems to be clear from the formulas, since as the base value gets larger the number of operations increases. Therefore we will need fewer iterations to reach the approximated value of up to 15 decimal places. 2. We see that among all base values appearing in the table, base 3 gives an approximation to the value of up to 15 decimal places with least number of cosines. So, we can say that the formula for base 3 is the best since it gives the value of up to 15 decimal places with minimal cost in terms of operations. Moreover, the odd base gives the approximation with fewer number of cosines compared to the even base. In general, as we go higher in the base (odd or even), the number of cosines increases. 8 Acknowledgement Thanks to my advisor Dr. Mohamed B. Rhouma for his continuous encouragement and support while writing this paper. I also thank Dr. Stefan Veldsman for his corrections and suggestions. References [1] M. Kac, Statistical Independence in Probability, Analysis and Number Theory, Carus Mathematical Monograph, No 12, The Mathematical Association of America, 1959. [2] K. E. Morrison, Cosine Products, Fourier Transforms and Random Sums, Amer. Math. Monthly, 102:716,1995. [3] L. Berggren, J. Borwein and P. Borwein, Pi: A Source Book, 2nd ed, Springer, 2000. [4] www-groups.dcs.st-and.ac.uk/~history/Mathematicians/Rademacher.html. 9