Understanding the Prime Number Theorem

Transcription

1 Understanding the Prime Number Theorem Misunderstood Monster or Beautiful Theorem? Liam Fowl September 5, 2014 Liam Fowl Understanding the Prime Number Theorem 1/20

2 Outline 1 Introduction 2 Complex Plane 3 Complex functions and Analytic Continuation 4 Gamma Function 5 Laplace Transform 6 Zeta Function 7 The Prime Number Theorem! Liam Fowl Understanding the Prime Number Theorem 2/20

3 Outline and Goals The Prime Number Theorem (PNT) Describes asymptotic behavior of π(x) Formally, π(x) x as x log(x) Goal Introduce preliminary topics necessary for the PNT Understand properties of functions necessary for PNT Briefly sketch proof of the PNT Liam Fowl Understanding the Prime Number Theorem 3/20

4 Outline 1 Introduction 2 Complex Plane 3 Complex functions and Analytic Continuation 4 Gamma Function 5 Laplace Transform 6 Zeta Function 7 The Prime Number Theorem! Liam Fowl Understanding the Prime Number Theorem 4/20

5 Complex Plane A complex number is a number of the form z = x + iy where z has both a real and imaginary component. Each complex number is an element in the complex plane (There is a one to one correspondance between C and R 2.) We can also talk about the extended complex plane C. Liam Fowl Understanding the Prime Number Theorem 5/20

6 Outline 1 Introduction 2 Complex Plane 3 Complex functions and Analytic Continuation 4 Gamma Function 5 Laplace Transform 6 Zeta Function 7 The Prime Number Theorem! Liam Fowl Understanding the Prime Number Theorem 6/20

7 Complex functions and Analytic Continuation Functions exist in C just like in normal Euclidean n space. We can talk about differentiating and integrating these functions. (Cauchy Integral formula seen below) f (a) = 1 2πi γ f (z) dz (1) z a We can also talk about something called analytic continuation. This means extending an analytic function from its normal domain of definition. Liam Fowl Understanding the Prime Number Theorem 7/20

8 Outline 1 Introduction 2 Complex Plane 3 Complex functions and Analytic Continuation 4 Gamma Function 5 Laplace Transform 6 Zeta Function 7 The Prime Number Theorem! Liam Fowl Understanding the Prime Number Theorem 8/20

9 Introduction Complex Plane Complex functions and Analytic Continuation Gamma Function Laplace Transform Zeta Function Gamma Function The Gamma function Γ(z) extends the factorial function to the complex plane Gamma function For Re(z) > 0, we have: Γ(z) = R 0 e t t z 1 dt (2) The identity Γ(z + 1) = zγ(z) arises from integration by parts. Using this identity, we can meromorphically extend Γ(z) to the rest of C. Liam Fowl Understanding the Prime Number Theorem 9/20

10 Introduction Complex Plane Complex functions and Analytic Continuation Gamma Function Laplace Transform Zeta Function Gamma Function contd Note: We can also express the Gamma function as an infinite product. Letting γ denote Euler s Constant, we have: Y z z 1 γz = ze (1 + )e k Γ(z) k k=1 Liam Fowl Understanding the Prime Number Theorem (3) 10/20

11 Outline 1 Introduction 2 Complex Plane 3 Complex functions and Analytic Continuation 4 Gamma Function 5 Laplace Transform 6 Zeta Function 7 The Prime Number Theorem! Liam Fowl Understanding the Prime Number Theorem 11/20

12 Introduction Complex Plane Complex functions and Analytic Continuation Gamma Function Laplace Transform Zeta Function Laplace Transform For a piecewise continuous function, h(s), the Laplace transform is defined as: R (Lh)(z) = 0 e sz h(s)ds (4) Aside InterestingR result: We can then write the derivative d Γ0(z) = 0 e sz g (s)ds Where g (s) = 1 es s dz Γ(z) Finally, we getqan asymptotic relationship for Gamma: Γ(z) = z z e z 2π (1 z z + O( n12 )) Liam Fowl Understanding the Prime Number Theorem 12/20

13 Outline 1 Introduction 2 Complex Plane 3 Complex functions and Analytic Continuation 4 Gamma Function 5 Laplace Transform 6 Zeta Function 7 The Prime Number Theorem! Liam Fowl Understanding the Prime Number Theorem 13/20

14 Introduction Complex Plane Complex functions and Analytic Continuation Gamma Function Laplace Transform Zeta Function The Zeta Function The Zeta function (Euler) is represented by P 1 ζ(s) = n=1 ns for Re(s) > 1 (5) We can see the more explicit connection of ζ(s) and the primes if we look at the infinite product representation of the zeta function: Q ζ(s) = p 1 p1 s for Re(s) > 1 (6) Liam Fowl Understanding the Prime Number Theorem 14/20

15 Introduction Complex Plane Complex functions and Analytic Continuation Gamma Function Laplace Transform Zeta Function Zeta Function contd Now we want to extend Zeta to the entire complex plane. How? A branch cut here... an Integral there... and a lot of magic. It turns out that the Zeta function can be meromorphically extended to the complex plane. It has one simple pole at s = 1. More formally, it satisfies the equation )Γ(1 s)ζ(1 s) ζ(s) = 2s π s 1 sin( πs 2 Liam Fowl Understanding the Prime Number Theorem (7) 15/20

16 Outline 1 Introduction 2 Complex Plane 3 Complex functions and Analytic Continuation 4 Gamma Function 5 Laplace Transform 6 Zeta Function 7 The Prime Number Theorem! Liam Fowl Understanding the Prime Number Theorem 16/20

17 Introduction Complex Plane Complex functions and Analytic Continuation Gamma Function Laplace Transform Zeta Function PNT Stated again, formally: The number of primes, π(x), not bigger than x satisfies π(x) x log (x) as x (8) The proof of the PNT is pretty messy (and magical according to Dr. Gamelin), but it relies heavily upon the following functions: P (9) (Re(s) > 1) Φ(s) = p logp(p) s P θ(x) = p x log (p) (10) Liam Fowl Understanding the Prime Number Theorem 17/20

18 Introduction Complex Plane Complex functions and Analytic Continuation Gamma Function Laplace Transform Zeta Function PNT contd First, the proof involves showing that ζ(s) does not have any zeros on the line Re(s) = 1. Essentially, the rest of the proof boils down to proving that θ(x) x, but to do that, we look at the Laplace transform of a nasty variation of θ(x) and a 1, tricky contour integral... and tada! we have that θ(x) x and by squeeze, we have the PNT. Interesting identity: π(x) Liam Fowl Understanding the Prime Number Theorem Rx 1 dt 2 log (t) 18/20

19 Acknowledgements A special thanks to the entire DRP program for this opportunity. Especially to Nathaniel Monson for putting up with my questions. Liam Fowl Understanding the Prime Number Theorem 19/20

20 Resources If you want to improve this style Nathaniel Monson s brain Complex Analysis, T. Gamelin Liam Fowl Understanding the Prime Number Theorem 20/20