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Using Origami to Find Rational Approximations of Irrational Roots Jeremy Lee Amherst Regional High School Hudson River Undergraduate Mathematics Conference Williams College April 6th, 2013 Origami Instructions • Folding paper allows us to find rational approximations of the square root of two. Side Unit fits Diagonal Once with remainder. Origami Instructions Side Unit fits Diagonal Once with remainder. Remainder fits side unit twice with remainder2 space Origami Instructions Side Unit fits Diagonal Once with remainder. Remainder fits side unit twice with remainder2 space Remainder2 fits Remainder unit twice with Remainder3 left over. Origami Instructions Summary of Origami Exercise • As we keep doing the exercise, you always get the number two after the first step. The next remainder will fit into previous remainder twice again. Summary of Origami Exercise • All these approximations of the square root of two come from the continued fractions representation of the square root of two. Continued Fractions Method 1 2 a0 1 a1 a2 1 a3 1 Continued Fractions Method 1 2 1 1 a1 a2 1 a3 1 Continued Fractions Method 1 2 1 1 2 a2 1 a3 1 Continued Fractions Method 1 2 1 1 2 2 1 a3 1 Continued Fractions Method 1 2 1 1 2 2 1 2 1 There will be twos continuing throughout the fraction after one. How to Use Continued Fractions to Build Approximations hi 2 1 2 1 1 2 2 1 2 1 2 3 1 7 5 2 1 2 1 2 1 2 17 12 1 2 ki Rational Approximations Of the Square Root of Two hi ki Denoting Parts of the Square Root of Two Continued Fraction 1 2 1 1 2 2 1 2 1 Let Big Box equal x What does the big box converge to? Algebra that Continued Fractions Converges to the Square Root of Two x 1 2 x 2 1 (1 2x x 1 2 2) x 2x 1 0 2 2 8 2 22 2 1 2 2 Why does the answer have to be 1 2? Assuming that this continued fraction converges, that is what x converges to. Therefore the continued fraction equals the square root of 2. The Babylonian Method Let’s try it with the square root of 2 Have x0 equal 1, our first approximate. x0 1 S x0 x n 1 1 S x n 2 xn x1 1 2 3 1 2 1 2 13 2 17 x2 3 2 2 12 2 1 17 2 577 x3 2 12 17 408 12 Algebra that Babylonian Method Converges to the Square Root of Two 1 2 x x 2 x x x 2 x 2 1 x x 2 1 2 2x x 2 2 x x 2 2 2 2 The reason why x is positive is because if the Babylonian Method converges , the result is what it will converge to. If x is negative then the result is the negative square root of two. There is a wonderful blog entry that is focused on the Babylonian Method and how it works with the square root of two. http://johncarlosbaez.wordpress.com/2011/12 /02/babylon-and-the-square-root-of-2/ hi ki hi+1/hi ki+1/ki What are the explicit formulas for each the numerator and the denominator of all rational approximations of the square root of two? There is almost a common ratio between consecutive numerators of the rational approximations. There is almost a common ratio between consecutive denominators of the rational approximations. The ratio is around 1 + sqrt(2) for both of these relationships. Maybe, hi and ki are the sums of two geometric sequences with a common ratio of 1 + sqrt(2) Rational Approximations Numerator 2 c r 1 h c 3 2 2 c r 3 2 h c 2 2 2 c 2 r 2 h1 c1 1 2 2 2 1 1 1 c1 1 c 2 r 2 r 2 5 h1 1 c 2 r 2 r 2 2 4 4 2 6 c 2 r 18 2 1 c1 c 2 1 3 2 3 c1 c 2 6 r r 2 2 3 2 c 2 r 17 6 h 2 c1 18 12 2 5 5 2 c 5 r 5 h 1 c r 2 r 7 5 2 c r 7 0 c r 2 r r h 4 c1 17 12 2 2 2 2 0 r 2r 1 2 r 1 2 h1 c1 1 c1 c 2 1 2 c1 c1 c 2 0 2 c2 1 2 c1 2c2 1 2c2 1 2 1 hi 1 2 1 2 i 1 2 1 2 i i hi Rational Approximations Denominator k 1 c 1 (1 2 ) c2r 1 k 2 c1 (3 2 2 ) c 2 r 2 2 c1 (1 2 ) c 2 (1 c1 c 2 c1 c 2 r 2 c 2 r 0 c 2 (1 k 3 c1 ( 7 5 2 ) c 2 r 3 5 5 k 1 c1 (5 5 2 ) 5 c 2 r 5 2 c1 2 c 2 r 5 c 2 r 0 3 r 1 2 2) 1 2 c2 c1 2 c1 2 c 2 r 4 c 2 r 0 2 2 ) c 2 (1 2 0 2 2c2 1 2 r 2r 1 0 c1 c 2 3 2 2 2 c 2 1 2 k 1 c1 ( 2 2 2 ) 2 c 2 r 2 2 2) 1 ki 4 2 4 2 4 1 2 i 2 4 1 2 i i hi ki Connection between Continued Fractions and Babylonian Method What happens when the continued fraction approximation is put into the Babylonian Method? hi 2 k i 2 1 4 1 8 1 2 1 2 2 k i 1 hi k i 2 2 2 hi k i hi 2 4 8 2i 2i 2 1 2 2 k i 2 1 4 2i 1 2 2 2 1 ( hi ) 2 ( k i ) 2 hi k i 1 1 i 1 1 1 8 1 2 4 2i 2 h2 i k 2i i 4 8 1 2 1 2 1 4 1 2 2i 2 1 i 2 2i 1 1 2 1 2 4 2i 1 2 2 1 hi 2 k i 1 ( hi ) 2 ( k i ) 2 ki h i 2 hi k i 2i 2 2 2i 1 2 1 1 2i 2 2i This is the Babylonian Method by taking a continued fraction approximation and average it with twice its reciprocal. k 2i 2i 2 h2 i h2 i k 2i The result is the continued fraction approximation of the square root of two, it’s just the number of steps of the continued fractions approximation is doubled for the Babylonian Method. You only did one step of the Babylonian Method. Conclusion •We did an origami experiment that finds the same rational approximations as the continued fractions method. •We found that the continued fraction and Babylonian Methods produced the same rational approximations for the square root of two. The Blog Entry http://johncarlosbaez.wordpress.com/2011/12/02/babylon-and-the-square-root-of-2/ Conclusion •We did an origami experiment that finds the same rational approximations as the continued fractions method. •We found that the continued fraction and Babylonian Methods produced the same rational approximations for the square root of two. •Could we do something similar with Fibonacci Numbers and the Golden Ratio which has the square root of five in it? The Blog Entry http://johncarlosbaez.wordpress.com/2011/12/02/babylon-and-the-square-root-of-2/