An Introduction to the Riemann Hypothesis
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An introduction to the Riemann hypothesis Author: Alexander Bielik [email protected] Supervisor: P¨arKurlberg SA104X { Degree Project in Engineering Physics Royal Institute of Technology (KTH) Department of Mathematics September 13, 2014 Abstract This paper exhibits the intertwinement between the prime numbers and the zeros of the Riemann zeta function, drawing upon existing literature by Davenport, Ahlfors, et al. We begin with the meromorphic continuation of the Riemann zeta function ζ and the gamma function Γ. We then derive a functional equation that relates these functions and formulate the Riemann hypothesis. We move on to the topic of finite-ordered functions and their Hadamard products. We show that the xi function ξ is of finite order, whence we obtain many useful properties. We then use these properties to find a zero-free region for ζ in the critical strip. We also determine the vertical distribution of the non-trivial zeros. We finally use Perron's formula to derive von Mangoldt's explicit formula, which is an approximation of the Cheby- shev function . Using this approximation, we prove the prime number theorem and conclude with an implication of the Riemann hypothesis. Contents Introduction 2 1 The statement of the Riemann hypothesis3 1.1 The Riemann zeta function ζ .........................................3 1.2 The gamma function Γ.............................................4 1.3 The functional equation............................................7 1.4 The critical strip................................................8 2 Zeros in the critical strip 10 2.1 Functions of finite order............................................ 10 2.2 The Hadamard product for functions of order 1............................... 11 2.3 Proving that ξ has order at most 1...................................... 13 2.4 A zero-free region for ζ ............................................. 16 2.5 The number of zeros in a rectangle...................................... 19 3 The distribution of prime numbers 23 3.1 Perron's formula for Dirichlet series...................................... 23 3.2 An approximation of the Chebyshev function ............................... 25 3.3 von Mangoldt's explicit formula........................................ 27 3.4 Proving the explicit formula.......................................... 28 3.5 The prime number theorem.......................................... 30 3.6 The smallest possible error term....................................... 32 Appendices 33 A Additional proofs 34 A.1 The Weierstrass form of Γ........................................... 34 A.2 Stirling's formula for ln Γ........................................... 35 A.3 Jensen's formula for holomorphic functions................................. 37 A.4 The uniqueness theorem for Dirichlet series................................. 38 Bibliography 39 1 Introduction In 1859, the German mathematician Georg Friedrich Bernhard Riemann proposed a hypothesis [Riemann, 1859, pp. 1-9] about prime numbers that would later bear his name, the Riemann hypothesis. The prime numbers do not appear to follow any obvious pattern. However, Riemann observed a close relation between the behavior of an elaborate function, the so-called Riemann zeta function, and the frequency of prime numbers. Riemann calculated a few zeros of this function, and quickly noted that the interesting ones lay on a certain vertical straight line in the complex plane. Riemann subsequently conjectured that all non-trivial zeros lie on this line. Today, over 1013 zeros are known [Gourdon, 2004, pp. 19-25], and all of them agree with the hypothesis. The Riemann hypothesis has important implications for the distribution of prime numbers and is strongly con- nected to the prime number theorem, which gives a good approximation of the density of prime numbers. In particular, the Riemann hypothesis gives a precise answer to how good the approximation given by the prime number theorem is. In a sense, the Riemann hypothesis conveys the idea that the prime numbers are distributed as regularly as possible. This regularity would tell a great deal about the average behavior of prime numbers in the long run. In today's society, the importance of prime numbers has increased rapidly, especially with the advance of in- formation technology and cryptography. However, the importance of the Riemann hypothesis goes far beyond its consequences for the distribution of prime numbers. It has been shown that hundreds of statements in number theory follow from it [Gowers et al., 2008, p. 715]. With this background, it might not be a surprise that some mathemati- cians consider the hypothesis to be the most important problem in pure mathematics, but it remains unresolved for now. The Riemann hypothesis is one of the seven Millennium Prize Problems that were stated by the Clay Mathe- matics Institute in 2000, carrying a million dollar prize for a correct solution. It is also part of the eighth problem in David Hilbert's list of unsolved problems. Other than the Riemann zeta function and its zeroes, there is currently no known approach to establish the distri- bution of prime numbers with desired precision. A disproof of the Riemann hypothesis would reveal a lot about how disordered the primes numbers really are. Enrico Bombieri, a prominent number theorist, remarked that "the failure of the Riemann hypothesis would create havoc in the distribution of prime numbers" [Havil, 2003, p. 205]. In 1770, Euler argued more pessimistically that "mathematicians have tried in vain to discover some order in the sequence of prime numbers but we have every reason to believe that there are some mysteries which the human mind will never penetrate" [Gowers et al., 2008, p. 348]. 2 1. The statement of the Riemann hypothesis 1.1 The Riemann zeta function ζ For complex numbers s with real part greater than 1, we define the Riemann zeta function by the absolutely convergent series 1 X 1 1 1 ζ(s) := = 1 + + + ··· : (1.1) ns 2s 3s n=1 By the Weierstrass M-test, we find that the convergence is uniform in the region Re(s) ≥ 1 + δ for any δ > 0. We now show that ζ is holomorphic within Re(s) > 1. To this end, we consider the sequence of holomorphic functions defined by i X 1 f (s) := : (1.2) i ns n=1 1 Since ffigi=1 converges uniformly to ζ on any compact subset of Re(s) > 1, it follows that the function is holomorphic there. In 1737, Euler deduced that 1 n Y 1 Y X 1 = 1 − p−s ps p2P p2P n=0 Y 1 1 = 1 + + + ··· ps p2s p2P X 1 X 1 = 1 + + + ··· ps psqs p2P p;q2P 1 1 1 X 1 = 1 + + + ··· = = ζ(s) (1.3) 2s 3s ns n=1 for any integer s > 1, where P denotes the set of prime numbers, though his argument can be extended to any complex number s with Re(s) > 1. In the above derivation, we first used the formula for a geometric series. Then, we rewrote the product and used the fundamental theorem of arithmetic, which states that each positive integer equals exactly one product of prime powers. This useful relation is usually called the Euler product formula. Some writers describe it as "the Golden Key" [Derbyshire, 2004, p. 105]. Taking the logarithm, we find that Y 1 X 1 ln = ln 1 − p−s 1 − p−s p2P p2P s X p = ln ps − 1 p2P X s s = ln jp j − ln jp − 1j p2P jpsj jpsj X dx X dx X 1 = ≤ < s ; (1.4) ˆjps−1j x ˆjps|−1 x jp j − 1 p2P p2P p2P 3 which clearly converges for Re(s) > 1. It follows that the product on the left-hand side of (1.3) converges. Hence, the formula (1.3) allows us to conclude that ζ has no zeros in the region Re(s) > 1, as each factor in the convergent product is different from zero. By partial summation, we can extend the domain of the function to Re(s) > 0: 1 X ζ(s) = n−s n=1 1 X = n n−s − (n + 1)−s n=1 1 X n+1 = s n x−s−1 dx ˆ n=1 n 1 = s bxcx−s−1 dx ˆ1 1 1 = s x−s dx − s fxgx−s−1 dx ˆ1 ˆ1 s 1 = − s fxgx−s−1 dx: (1.5) s − 1 ˆ1 Here, the symbols bxc and fxg = x − bxc stand for the integral and fractional parts of x, respectively. Since jfxgj ≤ 1, we see that the integral on the right of (1.5) converges absolutely in this extended domain. Furthermore, the convergence is uniform in the region Re(s) ≥ δ for any δ > 0. It follows that this new function is a meromorphic continuation of ζ. We observe that its only pole in this domain is a simple pole at s = 1 with residue Res(ζ; 1) = lim(s − 1)ζ(s) = 1: (1.6) s!1 We also observe that both terms in (1.5) are real and negative for real numbers 0 < s < 1, so ζ(s) < 0 on this line. Consequently, there are no zeros in this interval. For completeness, we mention another way to meromorphically continue ζ to Re(s) > 0. For Re(s) > 1, we observe that 1 1 1 2 X 1 X 2 X (−1)n−1 1 − ζ(s) = − = =: η(s); (1.7) 2s ns (2n)s ns n=1 n=1 n=1 whence ζ(s) = (1 − 21−s)−1η(s). Here, η is the Dirichlet eta function, which can be shown to converge for Re(s) > 0. 2πi While the right-hand side has singularities at s = 1+k ln 2 , where k 2 Z, only the singularity at s = 1 is non-removable. 1.2 The gamma function Γ The gamma function is an extension of the factorial function to complex numbers. For numbers s with positive real part, it is defined by the convergent integral 1 Γ(s) := ts−1e−t dt: (1.8) ˆ0 Let us show that this function is holomorphic. This time, we use Morera's theorem.