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 A159990 Coefficients in sexagesimal expansion of the positive root of x^3 + 2*x^2 + 10*x = 20, first studied by Leonardo of Pisa (Fibonacci) in 1225. 8
 1, 22, 7, 42, 33, 4, 38, 30, 50, 15, 43, 13, 56, 48, 24, 41, 0, 48, 22, 40, 39, 37, 23, 53, 55, 57, 45, 40, 5, 46, 50, 57, 28, 45, 46, 34, 2, 6, 7, 15, 25, 25, 13, 10, 59, 30, 13, 14, 7, 6, 15, 46, 23, 53, 59, 32, 24, 20, 11, 48, 35, 4, 4, 18, 33, 50, 7, 40, 16, 16, 1, 32, 24, 10, 43, 59, 23, 44, 51, 58, 11, 22, 26, 17 (list; constant; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Leonardo of Pisa (Fibonacci) found a(0), ..., a(5) but gave a(6) as 40. A159992(n)/A159993(n) = Sum_{k=0..n} a(k)/60^k = (Sum_{k=0..n} a(k)*60^(n-k))/60^n; let f(x) = x^3 + 2*x^2 + 10*x - 20, then for n > 0: a(n) = Max(k: f(A159992(n-1)/A159993(n - 1) + k/60^n)) < 0), a(n) + 1 = Min(k: f(A159992(n - 1)/A159993(n - 1) + k/60^n)) > 0); A159994(n)/A159995(n) = f(A159992(n)/A159993(n)). REFERENCES Cox, David A., Galois theory. Pure and Applied Mathematics (New York). Wiley-Interscience [John Wiley & Sons], Hoboken, NJ, 2004. xx+559 pp. ISBN: 0-471-43419-1 MR2119052 (2006a:12001). See page 9. A. F. Horadam, Eight hundred years young, The Australian Mathematics Teacher 31 (1975) 123-134. Alfred S. Posamentier & Ingmar Lehmann, The (Fabulous) Fibonacci Numbers. New York: Prometheus Books (2007): 21. LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..5000 Ezra Brown and Jason C. Brunson, Fibonacci's forgotten number Stanislaw Glushkov, On approximation methods of Leonardo Fibonacci, Historia Mathematica 3 (1976), pp. 291-296. J. J. O'Connor and E. F. Robertson, Fibonacci Clark Kimberling, Fibonacci [containing the Horadam article] Wikipedia, Sexagesimal EXAMPLE The root is 1 + 22/60 + 7/60^2 + 42/60^3 + 33/60^4 + 4/60^5 + 38/60^6 + 30/60^7 + 50/60^8 + ... Leonardo's approximation 1;22.7.42.33.4.40 is to be read as 1 + 22/60 + 7/60^2 + 42/60^3 + 33/60^4 + 4/60^5 + 40/60^6 = A159992(5)/A159993(5) + 40/60^6 = 1596577777 / 1166400000 ~= 1.3688081078532235 and f(1596577777/1166400000) ~= +6.7193226361369/10^10; compare this to A159992(6)/A159993(6) = A159992(5)/A159993(5) + 38/60^6 = 31931555539 / 23328000000 ~= 1.3688081078103566 and f(31931555539/23328000000) ~= -2.3239469709985/10^10. Assuming that Leonardo did similar calculations, the question may arise: why he didn't find a(6) = 38 instead of 40? Supposedly he just avoided the effort to calculate f(A159992(5)/A159993(5) + k/60^6) for k = 37, 38, or 39: 37/60^6 = 37/46656000000, 38/60^6 = 19/23328000000, or 39/60^6 = 13/15552000000; finally, he did calculate only f(A159992(5)/A159993(5) + k/60^6) for k = 36 and k = 40, the less complex cases concerning sexagesimal fractional arithmetic with 36/60^6 = 1/1296000000 and 40/60^6 = 1/1166400000: f(A159992(5)/A159993(5) + 36/60^6) ~= -1.9999999988632783, f(A159992(5)/A159993(5) + 40/60^6) ~= +0.0000000006719323. The latter result looks precise enough and could explain and justify Leonardo's 'rounding'. MATHEMATICA RealDigits[ Solve[x^3 + 2 x^2 + 10 x - 20 == 0, x][[3, 1, 2]], 60, 111][] (* Robert G. Wilson v, May 11 2012 *) PROG (PARI) polrootsreal(x^3+2*x^2+10*x-20) \\ Charles R Greathouse IV, Apr 14 2014 CROSSREFS Cf. A159991, A202300. Sequence in context: A040467 A194708 A069285 * A243629 A040466 A133682 Adjacent sequences:  A159987 A159988 A159989 * A159991 A159992 A159993 KEYWORD nonn,base,cons AUTHOR Reinhard Zumkeller, May 01 2009 STATUS approved

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Last modified September 15 20:11 EDT 2019. Contains 327086 sequences. (Running on oeis4.)