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A097546 Denominators of "Farey fraction" approximations to Pi. 6

%I

%S 0,1,1,1,1,1,2,3,4,5,6,7,8,15,22,29,36,43,50,57,64,71,78,85,92,99,106,

%T 113,219,332,445,558,671,784,897,1010,1123,1236,1349,1462,1575,1688,

%U 1801,1914,2027,2140,2253,2366,2479,2592,2705,2818,2931,3044,3157,3270

%N Denominators of "Farey fraction" approximations to Pi.

%C Given a real number x >= 1 (here x = Pi), start with 1/0 and 0/1 and construct the sequence of fractions f_n = r_n/s_n such that:

%C f_{n+1} = (r_k + r_n)/(s_k + s_n) where k is the greatest integer < n such that f_k <= x <= f_n. Sequence gives values s_n.

%C Write a 0 if f_n <= x and a 1 if f_n > x. This gives (for x = Pi) the sequence 1, 0, 0, 0, 1, 1, 1, 1, 0 (7 times), 1 (15 times, 0, 1,... Ignore the initial string 1, 0, 0, 0, which is always the same. Look at the runs lengths of the remaining sequence, which are in this case L_1 = 4, L_2 = 7, L_3 = 15, L_4 = 1, L_5 = 292, etc. (A001203). Christoffel showed that x has the continued fraction representation (L_1 - 1) + 1/(L_2 + 1/(L_3 + 1/(L_4 + ...))).

%D C. Brezinski, History of Continued Fractions and Padé Approximants, Springer-Verlag, 1991; pp. 151-152.

%D E. B. Christoffel, Observatio arithmetica, Ann. Math. Pura Appl., (II) 6 (1875), 148-153.

%H Dave Rusin, <a href="http://www.math.niu.edu/~rusin/known-math/99/farey">Farey fractions on sci.math</a> [Broken link]

%H Dave Rusin, <a href="/A002965/a002965.txt">Farey fractions on sci.math</a> [Cached copy]

%e The fractions are 1/0, 0/1, 1/1, 2/1, 3/1, 4/1, 7/2, 10/3, 13/4, 16/5, 19/6, 22/7, 25/8, 47/15, ...

%t f[x_, n_] := (m = Floor[x]; f0 = {m, m+1/2, m+1};

%t r = ({a___, b_, c_, d___} /; b < x < c) :> {b, (Numerator[b] + Numerator[c]) / (Denominator[b] + Denominator[c]), c}; Join[{m, m+1}, NestList[# /. r &, f0, n-3][[All, 2]]]); Join[{1, 0, 1, 2}, f[Pi, 52]] // Denominator (* _Jean-François Alcover_, May 18 2011 *)

%Y Cf. A097545.

%K nonn,frac,nice,easy

%O 0,7

%A _N. J. A. Sloane_, Aug 28 2004

%E Corrected and extended by _Joshua Zucker_, May 08 2006

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