# Understanding Farey Sequences and their Applications

Learn about the properties, construction, and theorems of Farey sequences. Discover the usefulness of these sequences in areas such as number theory and geometry with examples like Ford circles.

## About Understanding Farey Sequences and their Applications

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1. Farey Sequences and Applications Brandon Kriesten

2. Outline Definition of a Farey Sequence Definition of a Farey Sequence Properties of Farey Sequences Properties of Farey Sequences Construction of the Farey Sequence Construction of the Farey Sequence Theorem of two successive elements Theorem of two successive elements Number of Terms in a Farey Sequence Number of Terms in a Farey Sequence Applications of Farey Sequences Applications of Farey Sequences Ford Circles Ford Circles

3. Definition of a Farey Sequence A Farey Sequence is defined as all of the reduced fractions from [0,1] with denominators no larger than n, for all n in the domain of counting numbers, arranged in the order of increasing size. A Farey Sequence is defined as all of the reduced fractions from [0,1] with denominators no larger than n, for all n in the domain of counting numbers, arranged in the order of increasing size.

4. Some Examples of Farey Sequences F_1 = {0/1, 1/1} F_1 = {0/1, 1/1} F_2 = {0/1, 1/2, 1/1} F_2 = {0/1, 1/2, 1/1} F_3 = {0/1,1/3,1/2,2/3,1/1} F_3 = {0/1,1/3,1/2,2/3,1/1} F_7 = {0/1, 1/7, 1/6, 1/5, 1/4, 2/7, 1/3, 2/5, 3/7, 1/2, 4/7, 3/5, 2/3, 5/7, 3/4, 4/5, 5/6, 6/7, 1/1} F_7 = {0/1, 1/7, 1/6, 1/5, 1/4, 2/7, 1/3, 2/5, 3/7, 1/2, 4/7, 3/5, 2/3, 5/7, 3/4, 4/5, 5/6, 6/7, 1/1}

5. How to Construct the Farey Sequence To construct the Farey Sequence we must understand the concept of the mediant property. To construct the Farey Sequence we must understand the concept of the mediant property. The mediant property is stated as follows: The mediant property is stated as follows: Given a Farey Sequence F_(n) with the adjacent elements a/b < c/d, in the sequence F_(n+1) the elements will be mediated by the construction a/b < (a+c)/(b+d) < c/d if b+d ≤ n. If b+d > n the number will appear in a later sequence. Given a Farey Sequence F_(n) with the adjacent elements a/b < c/d, in the sequence F_(n+1) the elements will be mediated by the construction a/b < (a+c)/(b+d) < c/d if b+d ≤ n. If b+d > n the number will appear in a later sequence. Now from this definition and given F_2, construct F_3 and F4. Now from this definition and given F_2, construct F_3 and F4.

6. Theorem Theorem: Given a Farey sequence F_(n) with two successive elements (p_1)/(q_1) and (p_2)/(q_2), where (p_2)/(q_2) > (p_1)/(q_1), show that (p_2)/(q_2) - (p_1)/(q_1) = 1/(q_1*q_2). Theorem: Given a Farey sequence F_(n) with two successive elements (p_1)/(q_1) and (p_2)/(q_2), where (p_2)/(q_2) > (p_1)/(q_1), show that (p_2)/(q_2) - (p_1)/(q_1) = 1/(q_1*q_2).

7. Euler’s Totient Function To construct an equation for the number of elements in a Farey Sequence, we must first understand the concept of Euler’s Totient Function. Euler’s Totient function represents the number of positive integers less than a number n that are coprime to n. It is represented as the Greek symbol “phi” φ . To construct an equation for the number of elements in a Farey Sequence, we must first understand the concept of Euler’s Totient Function. Euler’s Totient function represents the number of positive integers less than a number n that are coprime to n. It is represented as the Greek symbol “phi” φ . E.g. φ (24) = 8, φ (6) = 2. E.g. φ (24) = 8, φ (6) = 2.

8. The number of Terms in a Farey Sequence The number of terms in a Farey Sequence is given by 1 plus the summation from k=1 to “n” of Euler’s Totient function. The number of terms in a Farey Sequence is given by 1 plus the summation from k=1 to “n” of Euler’s Totient function. This can also be written as the number of elements in F_(n-1) + φ (n). This can also be written as the number of elements in F_(n-1) + φ (n).

10. Ford Circles cont… A Ford Circle is defined as: For every rational number p/q where gcd(p,q) = 1, there exists a Ford Circle C(p,q) with a center (p/q, 1/2q^2) and a radius of 1/2q^2, that is tangent to the x axis at the point (p/q, 0). A Ford Circle is defined as: For every rational number p/q where gcd(p,q) = 1, there exists a Ford Circle C(p,q) with a center (p/q, 1/2q^2) and a radius of 1/2q^2, that is tangent to the x axis at the point (p/q, 0).

11. Theorem Theorem: The Largest Ford circle between tangent Ford circles. Suppose that C(a, b) and C(c, d) are tangent Ford circles. Then the largest Ford circle between them is C(a + c, b + d), the Ford circle associated with the mediant fraction. Theorem: The Largest Ford circle between tangent Ford circles. Suppose that C(a, b) and C(c, d) are tangent Ford circles. Then the largest Ford circle between them is C(a + c, b + d), the Ford circle associated with the mediant fraction.

12. For Further Research http://mathworld.wolfram.com/FareySequence.h tml http://mathworld.wolfram.com/FareySequence.h tml http://golem.ph.utexas.edu/category/2008/06/far ey_sequences_and_the_sternb.html http://golem.ph.utexas.edu/category/2008/06/far ey_sequences_and_the_sternb.html http://math.stanford.edu/circle/FareyFord.pdf http://math.stanford.edu/circle/FareyFord.pdf