First Converse, Then Inverse!

This post dedicated to the memory of Professor Tom M. Apostol on the occasion of the 100th anniversary of his birthday. For more details, see my paper here!

Converse of Elementary Functions and Their Fundamental Formulas:

First Formula of Radical:

$\boxed{(\sqrt x)^2=x}$

Second Formula of Radical:

$\boxed{\sqrt{x^2}=\begin{cases}x,~x\ge0\\-x,~x<0\end{cases}}$

First Formula of Cube Root Radical:

$\boxed{(\sqrt[3]x)^3=x}$

Second Formula of Cube Root Radical:

$\boxed{\sqrt[3]{x^3}=x}$

First Formula of Logarithm:

$\boxed{2^{\log_2(x)}=x}$

Second Formula of Logarithm:

$\boxed{\log_2(2^x)=x}$

First Formula of Arc Sine:

$\boxed{\sin(\text{Arcsin}(x))=x}$

Second Formula of Arc Sine:

$\boxed{\text{Arcsin}(\sin(x))=\begin{cases}x,~\text{first quadrant}\\\pi-x,~\text{second and third quadrants}\\x-2\pi,~\text{fourth quadrant}\end{cases}}$

First Formula of Arc Cosine:

$\boxed{\cos(\text{Arccos}(x))=x}$

Second Formula of Arc Cosine:

$\boxed{\text{Arccos}(\cos(x))=\begin{cases}x,~\text{first and second quadrants}\\2\pi-x,~\text{third and fourth quadrants}\end{cases}}$

First Formula of Arc Tangent:

$\boxed{\tan(\text{Arctan}(x))=x}$

Second Formula of Arc Tangent:

$\boxed{\text{Arctan}(\tan(x))=\begin{cases}x,~\text{first quadrant}\\x-\pi,~\text{second and third quadrants}\\x-2\pi,~\text{fourth quadrant}\end{cases}}$

Solutions to the book Amazing and Aesthetic Aspects of Analysis by Paul Loya

Part I: Some Standard Curriculum

Chapter 1: Very Naive Set Theory, Functions, and Proofs

The Algebra of Sets and the Language of Mathematics

Set Theory and Mathematical Statements

What Are Functions?

Chapter 2: Numbers, Numbers, and More Numbers

The Natural Numbers

The Principle of Mathematical Induction

The Integers

Primes and the Fundamental Theorem of Arithmetic

Decimal Representations of Integers

Real Numbers: Rational and “Mostly” Irrational

The Completeness Axiom of $\mathbb R$ and Its Consequences

Construction of the Real Numbers via Dedekind Cuts

$m$-Dimensional Euclidean Space

The Complex Number System

Cardinality and “Most” Real Numbers Are Irrational

Chapter 3: Infinite Sequences of Real and Complex Numbers

Convergence and $\epsilon-N$ Arguments for Limits of Sequences

A Potpourri of Limit Properties for Sequences

The Monotone Criteria, the Bolzano–Weierstrass Theorem, and $e$

Completeness, the Cauchy Criterion, and Contractive Sequences

Baby Infinite Series

Absolute Convergence and a Potpourri of Convergence Tests

Tannery’s Theorem and Defining the Exponential Function $\exp(z)$

Decimals and “Most” Real Numbers Are Irrational

Chapter 4: Limits, Continuity, and Elementary Functions

Continuity and $\epsilon-\delta$ Arguments for Limits of Functions

A Potpourri of Limit Properties for Functions

Continuity, Thomae’s Function, and Volterra’s Theorem

Compactness, Connectedness, and Continuous Functions

Amazing Consequences of Continuity

Monotone Functions and Their Inverses

Exponentials, Logs, Euler and Mascheroni, and the $\zeta$-Function

Proofs that $\sum1/p$ Diverges

Defining the Trig Functions and $\pi$, and Whichis Larger, $\pi^e$ or $e^\pi$?

Three Proofs of the Fundamental Theorem of Algebra (FTA)

The Inverse Trigonometric Functions and the Complex Logarithm

The Amazing $\pi$ and Its Computation from Ancient Times

Chapter 5: Some of the Most Beautiful Formulas in the World I–III

Beautiful Formulas I: Euler, Wallis, and Viète

Beautiful Formuls II: Euler, Gregory, Leibniz, and Madhava

Beautiful Formulas III: Euler’s Formula for $\zeta(2k)$

Part II: Extracurricular Activities

Chapter 6: Advanced Theory of Infinite Series

Summation by Parts, Bounded Variation, and Alternating Series

Lim Infs/Sups, Ratio/Roots, and Power Series

A Potpourri of Ratio-Type Tests and “Big $\mathcal O$” Notation

Pretty Powerful Properties of Power Series

Cauchy’s Double Series Theorem and A $\zeta$-Function Identity

Rearrangements and Multiplication of Power Series

Composition of Power Series and Bernoulli and Euler Numbers

The Logarithmic, Binomial, Arctangent Series, and $\gamma$

$\pi$, Euler, Fibonacci, Leibniz, Madhava, and Machin

Another Proof that ${\pi^2}/6=\sum_{n=1}^\infty1/{n^2}$ (The Basel Problem)

Chapter 7: More on the Infinite: Products and Partial Fractions

Introduction to Infinite Products

Absolute Convergence for Infinite Products

Euler and Tannery: Product Expansions Galore

Partial Fraction Expansions of the Trigonometric Functions

More Proofs that ${\pi^2}/6=\sum_{n=1}^\infty1/{n^2}$

Riemann’s Remarkable $\zeta$-Function, Probability, and ${\pi^2}/6$

Some of the Most Beautiful Formulas in the World IV

Chapter 8: Infinite Continued Fractions

Introduction to Continued Fractions

Some of the Most Beautiful Formulas in the World V

Recurrence Relations, Diophantus’s Tomb, and Shipwrecked Sailors

Convergence Theorems for Infinite Continued Fractions

Diophantine Approximations and the Mystery of $\pi$ Solved!

Continued Fractions, Calendars, and Musical Scales

The Elementary Functions and the Irrationality of $e^{p/q}$

Quadratic Irrationals and Periodic Continued Fractions

Archimedes’s Crazy Cattle Conundrum and Diophantine Equations

Epilogue: Transcendental Numbers, $\pi$, $e$, and Where’s Calculus?

Solutions to the book Galois Theory by Ian Stewart

Chapter 1: Classical Algebra

Chapter 2: The Fundamental Theorem of Algebra

Chapter 3: Factorisation of Polynomials

Chapter 4: Field Extensions

Chapter 5: Simple Extensions

Chapter 6: The Degree of an Extension

Chapter 7: Ruler-and-Compass Constructions

Chapter 8: The Idea Behind Galois Theory

Chapter 9: Normality and Separability

Chapter 10: Counting Principles

Chapter 11: Field Automorphisms

Chapter 12: The Galois Correspondence

Chapter 13: A Worked Example

Chapter 14: Solubility and Simplicity

Chapter 15: Solution by Radicals

Chapter 16: Abstract Rings and Fields

Chapter 17: Abstract Field Extensions

Chapter 18: The General Polynomial Equation

Chapter 19: Finite Fields

Chapter 20: Regular Polygons

Chapter 21: Circle Division

Chapter 22: Calculating Galois Groups

Chapter 23: Algebraically Closed Fields

Chapter 24: Transcendental Numbers

Chapter 25: What Did Galois Do or Know?

Solutions to the book Complex Variables by M. Ya. Antimirov, A. A. Kolyshkin and Remi Vaillancourt

Chapter 1: Functions of a Complex Variable

Complex numbers

Continuity in the complex plane

Functions of a complex variable

Analytic functions

Elementary analytic functions

Chapter 2: Elementary Conformal Mappings

Geometric meaning of f'(z)

Basic problems and principles of conformal mappings

Linear mapping and inversion

Linear fractional transformations

Symmetry and linear fractional transformations

Mapping by z^n and w = z^(1/n)

Exponential and logarithmic mappings

Mapping by Joukowsky’s function

Mapping by trigonometric functions

Chapter 3: Complex Integration and Cauchy’s Theorem

Paths in the complex plane

Complex line integrals

Cauchy’s Theorem

Cauchy’s integral formula and applications

Goursat’s Theorem

Chapter 4: Taylor and Laurent Series

Chapter 5: Singular Points and the Residue Theorem

Singular points of analytic functions

The residue theorem

Chapter 6: Elementary Definite Integrals

Rational functions over (-Infinity,+Infinity)

Rational functions times sine or cosine

Rational functions times exponential functions

Rational functions times a power of x

Chapter 7: Intermediate Definite Integrals

Rational functions over (0, +Infinity)

Forms containing (ln x)^p in the numerator

Forms containing In g(x) or arctan g(x)

Forms containing In in the denominator

Forms containing P(n)(e^x)/Q(m)(e^x)

Poisson’s integral

Fresnel integrals

Chapter 8: Advanced Definite Integrals

Rational functions times trigonometric functions

Forms containing (x^2 – 2a sinx + a^2)-1

Forms containing (h sin a.x + x cos a.x)^-1

Forms containing Bessel functions

Chapter 9: Further Applications of the Theory of Residues

Counting zeros and poles of meromorphic functions

The argument principle

Rouche’s Theorem

Simple-pole expansion of meromorphic functions

Infinite product expansion of entire functions

Chapter 10: Series Summation by Residues

Type of series considered

Summation of S1

Summation of S2

Summation of S3 and S4

Series with neither even nor odd terms

Series involving real zeros of entire functions

Series involving complex zeros of entire functions

Solutions to the book Introduction to Analytic Number Theory by Tom M. Apostol

Chapter 1: The Fundamental Theorem of Arithmetic

Chapter 2: Arithmetical Functions and Dirichlet Multiplication

Chapter 3: Averages of Arithmetical Functions

Chapter 4: Some Elementary Theorems on the Distribution of Prime Numbers

Chapter 5: Congruences

Chapter 6: Finite Abelian Groups and Their Characters

Chapter 7: Dirichlet’s Theorem on Primes in Arithmetic Progressions

Chapter 8: Periodic Arithmetical Functions and Gauss Sums

Chapter 9: Quadratic Residues and the Quadratic Reciprocity Law

Chapter 10: Primitive Roots

Chapter 11: Dirichlet Series and Euler Products

Chapter 12: The Functions $\zeta(s)$ and $L(s,\chi)$

Chapter 13: Analytic Proof of the Prime Number Theorem

Chapter 14: Partitions

One Finite Product

Solutions to the book Amazing and Aesthetic Aspects of Analysis by Paul Loya

Solutions to the book Galois Theory by Ian Stewart

Solutions to the book Complex Variables by M. Ya. Antimirov, A. A. Kolyshkin and Remi Vaillancourt

Solutions to the book Introduction to Analytic Number Theory by Tom M. Apostol

In this post, I wish to find the value of some finite product. I begin by defining a function $F_n(z)$ as follows and trying to find it’s roots

$\displaystyle F_n(z)=\frac1{2i}\left((1+\frac{iz}n)^n-(1-\frac{iz}n)^n\right)=$ $\displaystyle\frac{(-1)^m}{n^n}$ $\displaystyle z^n+\cdots+$ $\displaystyle1$ $\displaystyle z$

Let $z=n\tan(\theta)$ , then we have

$\displaystyle{1+\frac{iz}n=1+i\tan\theta=1+i\frac{\sin\theta}{\cos\theta}=\frac{1}{\cos\theta}(\cos\theta+i\sin\theta)=\sec\theta e^{i\theta}}$

and similarly $1-\frac{iz}n=\sec\theta e^{-i\theta}$ . So we have

$\displaystyle F_n(n\tan\theta)=\frac1{2i}\sec^n\theta(e^{in\theta}-e^{-in\theta})=\sec^n\theta\sin n\theta$

The sine function vanishes at integer multiples of $\pi$ , so it follows that $F_n(n\tan\theta)=0$ where $n\theta=k\pi$ for all integers $k$ , that is, for $\theta=k\pi/n$ for all $k\in\mathbb Z$ . Thus, $F_n(z_k)=0$ for

$\displaystyle z_k=n\tan\frac{k\pi}n=(2m+1)\tan\frac{k\pi}{2m+1}$

where we recall that $n=2m+1$ . Since $\tan\theta$ is strictly increasing on the interval $(-\pi/2,\pi/2)$ , it follows that

$\displaystyle{z_{-m}<z_{-m+1}<...<z_{-1}<z_0<z_1<...<z_{m-1}<z_m}$

moreover, since tangent is an odd function, we have $z_{-k}=-z_k$ for each $k$ . In particular we have found $2m+1=n$ distinct roots of $F_n(z)$ , so as a consequence of the fundamental theorem of algebra, we can write

$\displaystyle F_n(z)=$ $\displaystyle\frac{(-1)^m}{n^n}$ $\displaystyle{z\prod_{k=1}^m(z^2-z_k^2)=\frac{(-1)^m}{n^n}z\prod_{k=1}^m(z^2-n^2\tan^2\frac{k\pi}n)}$

$\displaystyle=\frac{(-1)^m}{n^n}\prod_{k=1}^m(-n^2\tan^2\frac{k\pi}n)z\prod_{k=1}^m\left(1-\frac{z^2}{n^2\tan^2\frac{k\pi}{n}}\right)$

$\displaystyle=$ $\displaystyle\frac1 n\prod_{k=1}^m\tan^2\frac{k\pi}{n}$ $\displaystyle z\prod_{k=1}^m\left(1-\frac{z^2}{n^2\tan^2\frac{k\pi}{n}}\right)$

So we have $\frac1{2m+1}\prod_{k=1}^m\tan^2\frac{k\pi}{2m+1}=1$ , and

$\boxed{\displaystyle\prod_{k=1}^m\tan\frac{k\pi}{2m+1}=\sqrt{2m+1}}$

More Elementary Proof for Euler’s Sine Expansion

Solutions to the book Amazing and Aesthetic Aspects of Analysis by Paul Loya

Solutions to the book Galois Theory by Ian Stewart

Solutions to the book Complex Variables by M. Ya. Antimirov, A. A. Kolyshkin and Remi Vaillancourt

Solutions to the book Introduction to Analytic Number Theory by Tom M. Apostol

Our goal in this post is to provide an elementary proof for the real version of Euler’s famous Sine expansion.

I first prove that for any $n$ that is a power of $2$ and for any $x\in\mathbb R$ ,

$\displaystyle{\sin(x)=n p_n(x)\sin\left(\frac x n\right)\cos\left(\frac x n\right),~p_n(x)=\prod_{k=1}^{\frac n 2-1}\left(1-\frac{\sin^2\frac x n}{\sin^2\frac{k\pi}n}\right)}~~~~~(1)$

by using the following more familiar identity

$\displaystyle\sin(x)=2\sin\frac x 2\sin\frac{\pi+x}2$

we can easily arrive to the following identity that is valid for $n$ equal to any power of $2$ ,

$\displaystyle\sin(x)=2^{n-1}\prod_{k=0}^{n-1}\sin\frac{k\pi+x}n$

$\displaystyle=2^{n-1}\sin\frac x n\sin\frac{\frac n 2\pi+x}n\prod_{k=1}^{\frac n 2-1}\sin\frac{k\pi+x}n\sin\frac{k\pi-x}n$

$\displaystyle=2^{n-1}\sin\frac x n\cos\frac x n\prod_{k=1}^{\frac n 2-1}\left(\sin^2\frac{k\pi}n-\sin^2\frac x n\right)~~~~~~~~~~(2)$

By considering what happens as $x\to0$ in the recent formula, we can obtain that for $n$ a power of $2$

$\displaystyle n=2^{n-1}\prod_{k=1}^{\frac n 2-1}\sin^2\frac{k\pi}n~~~~~~~~~~(3)$

and then I replace $(3)$ in $(2)$ and prove $(1)$ .

Now I choose $m<\frac n 2-1$ and break up $p_n(x)$ as follows

$\displaystyle p_n(x)=\prod_{k=1}^m\left(1-\frac{\sin^2\frac x n}{\sin^2\frac{k\pi}n}\right)\prod_{k=m+1}^{\frac n 2-1}\left(1-\frac{\sin^2\frac x n}{\sin^2\frac{k\pi}n}\right)~~~~~~~~~~(4)$

Since for $0<\theta<\frac{\pi}2$ , $\sin(\theta)>\frac2{\pi}\theta$ , we have

$\displaystyle\frac{\sin^2\frac x n}{\sin^2\frac{k\pi}n}<\frac{x^2}{4k^2}$

and if I choose $m$ and $n$ large enough such that $\frac{x^2}{4m^2}<1$ , then

$\displaystyle0<1-\frac{\sin^2\frac x n}{\sin^2\frac{k\pi}n}<1,k=m+1,...,\frac n 2-1$

which implies that

$\displaystyle0<\prod_{k=m+1}^{\frac n 2-1}\left(1-\frac{\sin^2\frac x n}{\sin^2\frac{k\pi}n}\right)<1$

Therefore from $(4)$ we have

$\displaystyle p_n(x)\le\prod_{k=1}^m\left(1-\frac{\sin^2\frac x n}{\sin^2\frac{k\pi}n}\right)$

Also we have

$\displaystyle{\prod_{k=m+1}^{\frac n 2-1}\left(1-\frac{\sin^2\frac x n}{\sin^2\frac{k\pi}n}\right)\ge1-\sum_{k=m+1}^{\frac n 2-1}\frac{\sin^2\frac x n}{\sin^2\frac{k\pi}n}\ge1-\frac{x^2}4\sum_{k=m+1}^\infty\frac1{k^2}=1-s_m}$

and once again by $(4)$ , we get

$\displaystyle p_n(x)\ge(1-s_m)\prod_{k=1}^m\left(1-\frac{\sin^2\frac x n}{\sin^2\frac{k\pi}n}\right)$

To summarize, I have shown that

$\displaystyle{(1-s_m)\prod_{k=1}^m\left(1-\frac{\sin^2\frac x n}{\sin^2\frac{k\pi}n}\right)\le p_n(x)\le\prod_{k=1}^m\left(1-\frac{\sin^2\frac x n}{\sin^2\frac{k\pi}n}\right)}~~~~~~~~~~(5)$

Now taking $n\to\infty$ in $(5)$ I arrive to

$\displaystyle(1-s_m)\prod_{k=1}^m\left(1-\frac{x^2}{k^2\pi^2}\right)\le\frac{\sin(x)}x\le\prod_{k=1}^m\left(1-\frac{x^2}{k^2\pi^2}\right)$

After rearrangement and then taking absolute values, we get

$\displaystyle\left|\frac{\sin(x)}x-\prod_{k=1}^m\left(1-\frac{x^2}{k^2\pi^2}\right)\right|\le|s_m|\left|\prod_{k=1}^m\left(1-\frac{x^2}{k^2\pi^2}\right)\right|$

but since we have

$\displaystyle{\left|\prod_{k=1}^m\left(1-\frac{x^2}{k^2\pi^2}\right)\right|\le\prod_{k=1}^m\left(1+\frac{x^2}{k^2\pi^2}\right)\le\prod_{k=1}^me^{\frac{x^2}{k^2\pi^2}}\le e^{\sum_{k=1}^\infty\frac{x^2}{k^2\pi^2}}=e^{\ell}}$

I can write

$\displaystyle\left|\frac{\sin(x)}x-\prod_{k=1}^m\left(1-\frac{x^2}{k^2\pi^2}\right)\right|\le|s_m|e^{\ell}$

Finally since $\lim_{m\to\infty}s_m=0$ , this inequality implies our goal, indeed

$\boxed{\displaystyle\prod_{k=1}^{\infty}\left(1-\frac{x^2}{k^2\pi^2}\right)=\frac{\sin(x)}x}$

One Finite Sum

Solutions to the book Amazing and Aesthetic Aspects of Analysis by Paul Loya

Solutions to the book Galois Theory by Ian Stewart

Solutions to the book Complex Variables by M. Ya. Antimirov, A. A. Kolyshkin and Remi Vaillancourt

Solutions to the book Introduction to Analytic Number Theory by Tom M. Apostol

In this post I want to find the value of the following finite sum,

$\displaystyle\sum_{k=1}^{m-1}\frac{1}{\sin^2(\frac{k\pi}{m})}$

I use the following amazing identity that you can see it’s proof in my previous posts

$\displaystyle\frac{1}{\sin^2(x)}=\sum_{n\in\mathbb{Z}}\frac{1}{(x+n\pi)^2}$

By this identity, I can write

$\displaystyle\sum_{k=0}^{m-1}\frac{1}{\sin^2(\frac{x+k\pi}{m})}=\sum_{k=0}^{m-1}\sum_{n\in\mathbb{Z}}\frac{1}{(\frac{x+k\pi}{m}+n\pi)^2}$

$\displaystyle=\sum_{k=0}^{m-1}\sum_{n\in\mathbb{Z}}\frac{1}{\frac{(x+k\pi+mn\pi)^2}{m^2}}$

$\displaystyle=m^2\sum_{k=0}^{m-1}\sum_{n\in\mathbb{Z}}\frac{1}{(x+k\pi+mn\pi)^2}$

$\displaystyle=m^2\sum_{n\in\mathbb{Z}}\sum_{k=0}^{m-1}\frac{1}{(x+(k+mn)\pi)^2}=\frac{m^2}{\sin^2(x)}$

and this follows that

$\displaystyle\sum_{k=1}^{m-1}\frac{1}{\sin^2(\frac{x+k\pi}{m})}=\frac{m^2}{\sin^2(x)}-\frac{1}{\sin^2(\frac{x}{m})}$

Hence,

$\boxed{\displaystyle\sum_{k=1}^{m-1}\frac{1}{\sin^2(\frac{k\pi}{m})}=\lim_{x\to0}\frac{m^2}{\sin^2(x)}-\frac{1}{\sin^2(\frac{x}{m})}=\frac{m^2-1}{3}}$

Evaluate Euler Sum!

Solutions to the book Amazing and Aesthetic Aspects of Analysis by Paul Loya

Solutions to the book Galois Theory by Ian Stewart

Solutions to the book Complex Variables by M. Ya. Antimirov, A. A. Kolyshkin and Remi Vaillancourt

Solutions to the book Introduction to Analytic Number Theory by Tom M. Apostol

Sums of the form $\displaystyle E(p,q)=\sum_{n=1}^\infty\frac{H_n^{(p)}}{n^q}$ where $\displaystyle H_n^{(p)}=\sum_{m=1}^n\frac{1}{m^p}$ , sometimes are called Euler sums. There are several ways to evaluate these sums. Aabout 240 years ago, Leonhard Euler(1707-1783) in 1775 proved that for $q\geq2$ ,

$\boxed{\displaystyle{\sum_{n=1}^\infty\frac{H_n}{n^q}=\frac{q+2}{2}\zeta(q+1)-\frac{1}{2}\sum_{r=1}^{q-2}\zeta(q-r)\zeta(r+1)}}$

In the following you can see an elementary proof of this formula in the three steps:

Step 1:

$\displaystyle\sum_{n=1}^\infty\frac{H_n}{n^q}=\sum_{n=1}^\infty\sum_{m=1}^n\frac{1}{mn^q}$

$\displaystyle=\sum_{m=1}^\infty\sum_{n=m}^\infty\frac{1}{mn^q}$

$\displaystyle=\zeta(q+1)+\sum_{m=1}^\infty\sum_{n=m+1}^\infty\frac{1}{mn^q}$

$\displaystyle=\zeta(q+1)+\sum_{m=1}^\infty\sum_{n=1}^\infty\frac{1}{m(n+m)^q}$

$\displaystyle=\zeta(q+1)+\frac{1}{2}\sum_{n=1}^\infty\sum_{m=1}^\infty\frac{1}{m(n+m)^q}+\frac{1}{n(n+m)^q}$

Hence,

$\boxed{\displaystyle{\sum_{n=1}^\infty\frac{H_n}{n^q}=\zeta(q+1)+\frac{1}{2}\sum_{n=1}^\infty\sum_{m=1}^\infty\frac{1}{m(n+m)^q}+\frac{1}{n(n+m)^q}}}$

Step 2:

$\displaystyle H_n=\sum_{m=1}^n\frac{1}{m}=\sum_{i=0}^{n-1}\sum_{m=1}^\infty\frac{1}{m+i}-\frac{1}{m+i+1}$

$\displaystyle=\sum_{m=1}^\infty\sum_{i=0}^{n-1}\frac{1}{m+i}-\frac{1}{m+i+1}$

$\displaystyle=\sum_{m=1}^\infty\frac{1}{m}-\frac{1}{n+m}$

Now by recent formula that is also an alternate definition of the Harmonic numbers, we can write:

$\boxed{\displaystyle\sum_{n=1}^\infty\frac{H_n}{n^q}=\sum_{n=1}^\infty\sum_{m=1}^\infty\frac{1}{mn^q}-\frac{1}{(n+m)n^q}}$

Step 3:

$\displaystyle\sum_{n=1}^\infty\frac{H_n}{n^q}$ $\displaystyle=2\sum_{n=1}^\infty\frac{H_n}{n^q}$ $\displaystyle-\sum_{n=1}^\infty\frac{H_n}{n^q}$

$\displaystyle{=2\zeta(q+1)+\sum_{n=1}^\infty\sum_{m=1}^\infty\frac{1}{m(n+m)^q}+\frac{1}{n(n+m)^q}-\sum_{n=1}^\infty\sum_{m=1}^\infty\frac{1}{mn^q}-\frac{1}{(n+m)n^q}}$

$\displaystyle=2\zeta(q+1)+\sum_{n=1}^\infty\sum_{m=1}^\infty\frac{1}{m(n+m)^{q-1}n}-\frac{1}{(n+m)n^{q-1}m}$

$\displaystyle=2\zeta(q+1)+\sum_{n=1}^\infty\sum_{m=n+1}^\infty\frac{1}{nm^{q-1}(m-n)}-\frac{1}{mn^{q-1}(m-n)}$

$\displaystyle=2\zeta(q+1)-\frac{1}{2}\sum_{\substack{m,n=1\\m\ne n}}^\infty\frac{1}{mn^{q-1}(m-n)}-\frac{1}{nm^{q-1}(m-n)}$

$\displaystyle=2\zeta(q+1)-\frac{1}{2}\sum_{\substack{m,n=1\\m\ne n}}^\infty\frac{m^{q-2}-n^{q-2}}{m^{q-1}n^{q-1}(m-n)}$

$\displaystyle=2\zeta(q+1)-\frac{1}{2}\sum_{\substack{m,n=1\\m\ne n}}^\infty\sum_{r=1}^{q-2}\frac{1}{m^{q-r}n^{r+1}}$

$\displaystyle=2\zeta(q+1)-\frac{1}{2}\left(\sum_{m,n=1}^\infty\sum_{r=1}^{q-2}\frac{1}{m^{q-r}n^{r+1}}-\sum_{\substack{m,n=1\\m=n}}^\infty\sum_{r=1}^{q-2}\frac{1}{m^{q-r}n^{r+1}}\right)$

$\displaystyle=2\zeta(q+1)+\frac{q-2}{2}\zeta(q+1)-\frac{1}{2}\sum_{r=1}^{q-2}\sum_{m=1}^\infty\sum_{n=1}^\infty\frac{1}{m^{q-r}n^{r+1}}$

$\displaystyle{=\frac{q+2}{2}\zeta(q+1)-\frac{1}{2}\sum_{r=1}^{q-2}\zeta(q-r)\zeta(r+1)}.$

Another Result By the Flajolet-Vardi Theorem

Solutions to the book Amazing and Aesthetic Aspects of Analysis by Paul Loya

Solutions to the book Galois Theory by Ian Stewart

Solutions to the book Complex Variables by M. Ya. Antimirov, A. A. Kolyshkin and Remi Vaillancourt

Solutions to the book Introduction to Analytic Number Theory by Tom M. Apostol

By using the Flajolet-Vardi theorem we can find the value of the another amazing convergent series. Indeed, following series

$\displaystyle\sum_{n=2}^{\infty}\frac{\zeta(n)}{k^n}$

I first recall the Flajolet-Vardi theorem that you can find it’s proof in my second post:

Flajolet-Vardi Theorem:

If $\displaystyle f\left(z \right)=\sum_{n=2}^{\infty}a_{n}z^n$ and $\displaystyle\sum_{n=2}^{\infty}|a_n|$ converges then‎,

$\displaystyle\sum_{n=1}^{\infty}f\left(\frac{1}{n}\right)=\sum_{n=2}^{\infty}a_n\zeta\left(n\right).$

This theorem shows that $\displaystyle\sum_{n=2}^{\infty}\frac{\zeta(n)}{k^n}=\sum_{n=1}^{\infty}\frac{1}{kn(kn-1)}$ , because if we let $f(z)=\displaystyle\sum_{n=2}^{\infty}\frac{z^n}{k^n}$ , then $f(z)=\frac{z^2}{k(k-z)}$ and by this theorem

$\displaystyle{\sum_{n=2}^\infty\frac{\zeta(n)}{k^n}=\sum_{n=1}^\infty f\left(\frac{1}{n}\right)=\sum_{n=1}^\infty\frac{1}{kn(kn-1)}}$

Therefore now we must find the value of $\displaystyle\sum_{n=1}^\infty\frac{1}{kn(kn-1)}$ . we use the Taylor expansion of $\log(1-x)$ and the fact that the sum $\displaystyle\sum_{\alpha^k=1}\alpha^n$ is $k$ if $k$ divides $n$ and $0$ otherwise.

There are some $\log$ ‘s of complex numbers. Those numbers have always non-negative real part, for the Argument we take the angle between $\displaystyle\frac{-\pi}{2}$ and $\displaystyle\frac{\pi}{2}$ , so that it fits with the power series for $\log(1-x)$ .

$\displaystyle{\sum_{n=1}^\infty \frac{x^{kn}}{kn}=\frac{-\log(1-x^k)}{k}=\frac{-1}{k}\sum_{\alpha^k=1}\log(1-\alpha x)}$

$\displaystyle\sum_{\alpha^k=1}\sum_{m=1}^\infty\frac{\alpha(\alpha x)^m}{m}=k\sum_{n=1}^{\infty}\frac{x^{kn-1}}{(kn-1)}$

but also

$\displaystyle{\sum_{\alpha^k=1}\sum_{m=1}^\infty\frac{\alpha(\alpha x)^m}{m}=-\sum_{\alpha^k=1}\alpha\log(1-\alpha x)}.$

We thus have

$\displaystyle{\sum_{n=1}^\infty\frac{x^{kn}}{kn(kn-1)}=\sum_n x^{kn}\left(\frac{1}{kn-1}-\frac{1}{kn}\right)}$

$\displaystyle{=\frac{1}{k}\sum_{\alpha^k=1}(1-x\alpha)\log(1-\alpha x)}.$

We have to take the limit $x\to 1$ . The $\alpha=1$ term disappears, so we get

$\displaystyle{\sum_{n=1}^\infty\frac{1}{kn(kn-1)}=\frac{1}{k}\sum_{\alpha^k=1,\alpha\neq1}(1-\alpha)\log(1-\alpha),\alpha=e^{\frac{2\pi im}{k}}}$

$\displaystyle{=\frac{1}{k}\sum_{m=1}^{k-1}\left(1-\cos\frac{2\pi m}{k}-i\sin\frac{2\pi m}{k}\right)\left(\log\left(2\sin\frac{\pi m}{k}\right)+\pi i\left(\frac{m}{k}-\frac{1}{2}\right)\right)}$

$\displaystyle{=\frac{1}{k}\sum_{m=1}^{k-1}\left[\left(1-\cos\frac{2\pi m}{k}\right)\log\left(2\sin\frac{\pi m}{k}\right)+\frac{(2m-k)\pi}{2k}\sin\frac{2\pi m}{k}\right]}.$

Examples:

For $k=2$ we have

$\displaystyle{\sum_{n=2}^\infty\frac{\zeta(n)}{2^n}=\sum_{n=1}^\infty\frac{1}{2n(2n-1)}=\frac{1}{2}\left[(1-\cos\pi)\log\left(2\sin\frac{\pi}{2}\right)\right]=\log(2)}.$

and also for $k=3$ ,

$\displaystyle\sum_{n=2}^\infty\frac{\zeta(n)}{3^n}=\sum_{n=1}^\infty\frac{1}{3n(3n-1)}$

$\displaystyle=\frac{1}{3}\left[\left(1-\cos\frac{2\pi}{3}\right)\log\left(2\sin\frac{\pi}{3}\right)-\frac{\pi}{6}\sin\frac{2\pi}{3}+\left(1-\cos\frac{4\pi}{3}\right)\log\left(2\sin\frac{2\pi}{3}\right)+\frac{\pi}{6}\sin\frac{4\pi}{3}\right]$

$\displaystyle=\frac{1}{2}\log(3)-\frac{\pi}{6\sqrt{3}}.$

Calculating an Infinite Sums

Solutions to the book Amazing and Aesthetic Aspects of Analysis by Paul Loya

Solutions to the book Galois Theory by Ian Stewart

Solutions to the book Complex Variables by M. Ya. Antimirov, A. A. Kolyshkin and Remi Vaillancourt

Solutions to the book Introduction to Analytic Number Theory by Tom M. Apostol

In this post I want to generalize the given formula at my second post as follows

$\boxed{\displaystyle\sum_{n=1}^\infty\frac{(m-1)^n-1}{m^n}\zeta(n+1)=\pi\cot\frac{\pi}{m}}$

To prove this , I first prove following useful identity

$\displaystyle\frac{1}{\sin^2x}=\sum_{n\in\mathbb{Z}}\frac{1}{(x+n\pi)^2}$

PROOF: (Trigonometric Method) Note that

$\displaystyle{\frac{1}{\sin^2x}=\frac{1}{4\sin^2\frac{x}{2}\cos^2\frac{x}{2}}=\frac{1}{4}\left(\frac{1}{\sin^2\frac{x}{2}}+\frac{1}{\cos^2\frac{x}{2}}\right)=\frac{1}{4}\left(\frac{1}{\sin^2\frac{x}{2}}+\frac{1}{\sin^2\frac{\pi+x}{2}}\right)}$

$\displaystyle=\frac{1}{4^2}\left(\frac{1}{\sin^2\frac{x}{2^2}}+\frac{1}{\sin^2\frac{2\pi+x}{2^2}}+\frac{1}{\sin^2\frac{\pi+x}{2^2}}+\frac{1}{\sin^2\frac{3\pi+x}{2^2}}\right)$

Repeatedly applying $\displaystyle\frac{1}{\sin^2x}=\frac{1}{4}\left(\frac{1}{\sin^2\frac{x}{2}}+\frac{1}{\sin^2\frac{\pi+x}{2}}\right)$ , we arrive at the following formula:

$\displaystyle\frac{1}{\sin^2x}=\frac{1}{4^k}\sum_{n=0}^{2^k-1}\frac{1}{\sin^2\frac{x+n\pi}{2^k}}$

but

$\displaystyle{\frac{1}{4^k}\sum_{n=0}^{2^k-1}\frac{1}{\sin^2\frac{x+n\pi}{2^k}}=\frac{1}{4^k}\sum_{n=-2^{k-1}}^{2^{k-1}-1}\frac{1}{\sin^2\frac{x+n\pi}{2^k}}=\lim_{k\to\infty}\sum_{n=-k}^k\frac{1}{(x+n\pi)^2}=\sum_{n\in\mathbb{Z}}\frac{1}{(x+n\pi)^2}}$

and now note that

$\displaystyle\sum_{n\in\mathbb{Z}}\frac{1}{x+n}=\pi\cot\pi x$

Because,

$\displaystyle{\cot x=-\int\frac{1}{\sin^2x}dx=-\int\sum_{n\in\mathbb{Z}}\frac{1}{(x+n\pi)^2}dx=\sum_{n\in\mathbb{Z}}\frac{1}{x+n\pi}}$

By using this recent identity we can write

$\displaystyle\sum_{n=1}^\infty\frac{(m-1)^n-1}{m^n}\zeta(n+1)$ $\displaystyle=\sum_{n=1}^\infty\sum_{k=1}^\infty\frac{(m-1)^n-1}{m^n}\frac1{k^{n+1}}$

$\displaystyle=\sum_{k=1}^\infty\frac1k\sum_{n=1}^\infty\left(\frac{(m-1)^n}{m^nk^n}-\frac1{m^nk^n}\right)$

$\displaystyle=\sum_{k=1}^\infty\frac1k\left(\frac{\frac{m-1}{mk}}{1-\frac{m-1}{mk}}-\frac{\frac1{mk}}{1-\frac1{mk}}\right)$

$\displaystyle=\sum_{k=1}^\infty\left(\frac1{\frac1m-k}+\frac1{\frac1m+k-1}\right)$

$\displaystyle=\sum_{k\in\mathbb{Z}}\frac1{\frac1m+k}$ $\displaystyle=\pi\cot\frac{\pi}{m}.$

The Basel Problem, Double Integral Method II

Solutions to the book Amazing and Aesthetic Aspects of Analysis by Paul Loya

Solutions to the book Galois Theory by Ian Stewart

Solutions to the book Complex Variables by M. Ya. Antimirov, A. A. Kolyshkin and Remi Vaillancourt

Solutions to the book Introduction to Analytic Number Theory by Tom M. Apostol

One another way that can evaluate $\zeta(2)$ by double integral is to write $\zeta(2)$ as follows

$\displaystyle\zeta(2)=\frac{4}{3}\sum_{n=0}^{\infty}\frac{1}{(2n+1)^2}$

This is true, since we have

$\displaystyle{\zeta(2)=\sum_{n=1}^{\infty}\frac{1}{n^2}=\sum_{n=1}^{\infty}\frac{1}{(2n)^2}+\sum_{n=0}^{\infty}\frac{1}{(2n+1)^2}=\frac{1}{4}\sum_{n=1}^{\infty}\frac{1}{n^2}+\sum_{n=0}^{\infty}\frac{1}{(2n+1)^2}}.$

Now integrating by parts yields

$\displaystyle{\int_0^1(-x^{2n}\ln(x))dx=\left[-\frac{x^{2n+1}}{2n+1}\ln(x)\right]_0^1+\int_0^1\frac{x^{2n}}{2n+1}dx=\frac{1}{(2n+1)^2}}$

and by using the monotone convergence theorem, I can write

$\displaystyle{\zeta(2)=\frac{4}{3}\sum_{n=0}^{\infty}\frac{1}{(2n+1)^2}=\frac{4}{3}\sum_{n=0}^{\infty}\int_0^1(-x^{2n}\ln(x))dx=\frac{4}{3}\int_0^1\frac{\ln(x)}{x^2-1}dx}$

Hence,

$\displaystyle\zeta(2)=\frac{4}{3}\int_0^1\frac{\ln(x)}{x^2-1}dx$

$\displaystyle=\frac{4}{3}\times\frac{1}{2}\left(\int_0^1\frac{\ln(u)}{u^2-1}du+\int_0^1\frac{\ln(v)}{v^2-1}dv\right)$

$\displaystyle=\frac{2}{3}\left(\int_0^1\frac{\ln(u)}{u^2-1}du+\int_1^{\infty}\frac{\ln(v)}{v^2-1}dv\right)$

$\displaystyle=\frac{2}{3}\left(\int_0^1\frac{\ln(u)}{u^2-1}du+\int_0^{\infty}\frac{\ln(u)}{u^2-1}du-\int_0^{1}\frac{\ln(u)}{u^2-1}du\right)$

$\displaystyle=\frac{2}{3}\int_0^{\infty}\frac{\ln(u)}{u^2-1}du=\frac{2}{3}\times\frac{1}{2}\int_0^{\infty}\frac{\ln(u^2)}{u^2-1}du$

$\displaystyle=\frac{1}{3}\int_0^{\infty}\frac{1}{1-u^2}\ln(\frac{1}{u^2})du=\frac{1}{3}\int_0^{\infty}\frac{1}{1-u^2}\left[\ln\left(\frac{1+v}{1+u^2v}\right)\right]_{v=0}^{v=\infty}du$

$\displaystyle=\frac{1}{3}\int_0^{\infty}\frac{1}{1-u^2}\left(\int_0^{\infty}\left(\frac{1}{1+v}-\frac{u^2}{1+u^2v}\right)dv\right)du$

$\displaystyle{=\frac{1}{3}\int_0^{\infty}\int_0^{\infty}\frac{1}{(1+v)(1+u^2v)}dv\,du=\frac{1}{3}\int_0^{\infty}\frac{1}{1+v}\int_0^{\infty}\frac{1}{1+u^2v}du\,dv}$

$\displaystyle{=\frac{1}{3}\int_0^{\infty}\left(\frac{1}{1+v}\left[\frac{\arctan(\sqrt{v}u)}{\sqrt{v}}\right]_{u=0}^{u=\infty}\right)dv=\frac{1}{3}\times\frac{\pi}{2}\int_0^{\infty}\frac{1}{\sqrt{v}(1+v)}dv}$

$\displaystyle{=\frac{\pi}{3}\int_0^{\infty}\frac{1}{1+w^2}dw=\frac{\pi}{3}\left[\arctan(w)\right]_0^{\infty}=\frac{\pi}{3}\times\frac{\pi}{2}=}$ $\displaystyle{\frac{\pi^2}{6}}.$

The Basel Problem, Double Integral Method I

Solutions to the book Amazing and Aesthetic Aspects of Analysis by Paul Loya

Solutions to the book Galois Theory by Ian Stewart

Solutions to the book Complex Variables by M. Ya. Antimirov, A. A. Kolyshkin and Remi Vaillancourt

Solutions to the book Introduction to Analytic Number Theory by Tom M. Apostol

The Italian mathematician Pietro Mengoli (1625–1686), in his 1650 book Novae quadraturae arithmeticae, seu de additione fractionum, posed the following question: What’s the value of the sum

$\zeta(2)=\displaystyle\sum_{n=1}^{\infty}\frac{1}{n^2}$

Here’s what Mengoli said:

Having concluded with satisfaction my consideration of those arrangements of fractions, I shall move on to those other arrangements that have the unit as numerator, and square numbers as denominators. The work devoted to this consideration has bore some fruit — the question itself still awaiting solution — but it [the work] requires the support of a richer mind, in order to lead to the evaluation of the precise sum of the arrangement [of fractions] that I have set myself as a task.

The task of finding the sum was made popular through Jacob Bernoulli (1654–1705) when he wrote about it in 1689, and was solved by Leonhard Euler (1707–1783) in 1735. Bernoulli was so baffled by the unknown value of the series that he wrote:

If somebody should succeed in finding what till now withstood our efforts and communicate it to us, we shall be much obliged to him.

Before Euler’s solution to this request, known as the Basel problem (Bernoulli lived in Basel, Switzerland), this problem had eluded many of the great mathematicians of that day: In 1742, Euler wrote:

Jacob Bernoulli does mention those series, but confesses that, in spite of all his efforts, he could not get through, so that Joh. Bernoulli, de Moivre, and Stirling, great authorities in such matters, were highly surprised when I told them that I had found the sum of $\displaystyle\zeta(2)$ , and even of $\displaystyle\zeta(n)$ for $n$ even.

Needless to say, it shocked the mathematical community when Euler found the sum to be $\displaystyle\frac{\pi^2}{6}$ ; in the introduction to his famous 1735 paper De summis serierum reciprocarum (On the sums of series of reciprocals), where he first proves that $\displaystyle\zeta(2)=\frac{\pi^2}{6}$ , Euler writes:

So much work has been done on the series $\displaystyle\zeta(n)$ that it seems hardly likely that anything new about them may still turn up . . . I, too, in spite of repeated efforts, could achieve nothing more than approximate values for their sums . . . Now, however, quite unexpectedly, I have found an elegant formula for $\displaystyle\zeta(2)$ , depending on the quadrature of the circle [i.e., upon $\pi$ ].

In page 122 of the book “Topics in Number Theory”(1956), by William J. LeVeque there is an exercise for evaluating the following integral in two ways.

$\displaystyle\int_0^1\!\!\!\int_0^1\frac1{1-xy}\,dy\,dx$

First way is to write the integrand as a geometric series,

$\displaystyle\int_0^1\!\!\!\int_0^1\frac1{1-xy}\,dy\,dx=\int_0^1\!\!\!\int_0^1\left(\sum_{n=1}^\infty(xy)^{n-1}\right)\,dy\,dx=\sum_{n=1}^\infty\frac1{n^2}$

and the second way by use of a suitable change of variables

( $y:=u-v,x:=u+v$ )

which is also published by Tom M. Apostol in 1983 in Mathematical Intelligencer.

Note that

$\displaystyle{\frac{1}{n^2}}=\displaystyle\int_0^1\displaystyle\int_0^1x^{n-1}y^{n-1}dx\,dy$

and by the monotone convergence theorem we get

$\displaystyle\sum_{n=1}^{\infty}\frac{1}{n^2}=\displaystyle\int_0^1\displaystyle\int_0^1\left(\sum_{n=1}^{\infty}(xy)^{n-1}\right)dx\,dy$

$=\displaystyle\int_0^1\displaystyle\int_0^1\frac{1}{1-xy}dx\,dy$

We change variables in this by putting $(u,v)=\left(\frac{x+y}{2},\frac{y-x}{2}\right)$ , so that $(x,y)=(u-v,u+v)$ . Hence

$\zeta$ $(2)$ $=2\displaystyle\iint_S\frac{du\,dv}{1-u^2+v^2}$

where $S$ is the square with vertices $(0,0), \left(\frac{1}{2},\frac{-1}{2}\right), (1,0)$ and $(\frac{1}{2},\frac{1}{2})$ .

Exploiting the symmetry of the square we get

$\zeta$ $(2)$ $=4\displaystyle\int_0^{\frac{1}{2}}\displaystyle\int_0^u\frac{dv\,du}{1-u^2+v^2}+4\displaystyle\int_{\frac{1}{2}}^1\displaystyle\int_0^{1-u}\frac{dv\,du}{1-u^2+v^2}$

$=4\displaystyle\int_0^{\frac{1}{2}}\frac{\tan^{-1}\left(\frac{u}{\sqrt{1-u^2}}\right)}{\sqrt{1-u^2}}du+4\displaystyle\int_{\frac{1}{2}}^1\frac{\tan^{-1}\left(\frac{1-u}{\sqrt{1-u^2}}\right)}{\sqrt{1-u^2}}du$

Now $\tan^{-1}\left(\frac{u}{\sqrt{1-u^2}}\right)=\sin^{-1}(u)$ , and if $\alpha=\tan^{-1}\left(\frac{1-u}{\sqrt{1-u^2}}\right)$ then $\tan^2(\alpha)=\frac{1-u}{1+u}$ and $\sec^2(\alpha)=\frac{2}{1+u}$ .

It follows that $u=2\cos^2(\alpha)-1=\cos(2\alpha)$ and so $\alpha=\frac{1}{2}\cos^{-1}(u)=\frac{\pi}{4}-\frac{\sin^{-1}(u)}{2}$ . Hence

$\zeta$ $(2)$ $=4\displaystyle\int_0^{\frac{1}{2}}\frac{\sin^{-1}(u)}{\sqrt{1-u^2}}du+4\displaystyle\int_{\frac{1}{2}}^1\frac{\frac{\pi}{4}-\frac{\sin^{-1}(u)}{2}}{\sqrt{1-u^2}}du$

$\displaystyle=\left[2(\sin^{-1}(u))^2\right]_0^{\frac{1}{2}}+\left[\pi\sin^{-1}(u)-(\sin^{-1}(u))^2\right]_{\frac{1}{2}}^1$

$\displaystyle=\frac{\pi^2}{18}+\frac{\pi^2}{2}-\frac{\pi^2}{4}-\frac{\pi^2}{6}+\frac{\pi^2}{36}=$ $\displaystyle\frac{\pi^2}{6}.$

Flajolet-Vardi Theorem and Calculating Infinite Sums

Solutions to the book Amazing and Aesthetic Aspects of Analysis by Paul Loya

Solutions to the book Galois Theory by Ian Stewart

Solutions to the book Complex Variables by M. Ya. Antimirov, A. A. Kolyshkin and Remi Vaillancourt

Solutions to the book Introduction to Analytic Number Theory by Tom M. Apostol

In this post I want to find the value of the sum

$\displaystyle\sum_{n=1}^{\infty}\frac{3^n-1}{4^n} \zeta \left(n+1 \right)$

Note that for $s>1$ , $\zeta(s)=\displaystyle\sum_{n=1}^{\infty}\frac{1}{n^s}$ .

I use following formula that is called Gregory-Leibniz-Madhava’s series

$\displaystyle\sum_{n=1}^{\infty}\frac{1}{4n-3}-\frac{1}{4n-1}=\frac{\pi}{4}$

If we define $f$ function as follows:

$\displaystyle{f\left(z\right)=\frac{z}{4-3z}-\frac{z}{4-z}}$

then

$\displaystyle{f(z)=\frac{\frac{z}{4}}{1-\frac{3z}{4}}-\frac{\frac{z}{4}}{1-\frac{z}{4}}=\frac{z}{4}\sum_{n=0}^{\infty}\left(\frac{3z}{4}\right)^n-\left(\frac{z}{4}\right)^n=\frac{z}{4}\sum_{n=1}^{\infty}\left(\frac{3z}{4}\right)^n-\left(\frac{z}{4}\right)^n}$

$\displaystyle{=\frac{z}{4}\sum_{n=2}^{\infty}\left(\frac{3z}{4}\right)^{n-1}-\left(\frac{z}{4}\right)^{n-1}=\frac{1}{4}\sum_{n=2}^{\infty}\left[\left(\frac{3}{4}\right)^{n-1}-\left(\frac{1}{4}\right)^{n-1}\right]z^n=\frac{1}{4}\sum_{n=2}^{\infty}\frac{3^{n-1}-1}{4^{n-1}}z^n}.$

Now I use this theorem

THEOREM (Flajolet–Vardi): If $f\left(z \right)=\displaystyle\sum_{n=2}^{\infty}a_{n}z^n$ and $\displaystyle\sum_{n=2}^{\infty}|a_n|$ converges then,

$\displaystyle\sum_{n=1}^{\infty}f\left(\frac{1}{n}\right)=\displaystyle\sum_{n=2}^{\infty}a_n$ $\zeta$ $\left(n\right)$ .

PROOF:

Because $\displaystyle\sum_{m=2}^{\infty}|a_m|<\infty$ ,

$\displaystyle\sum_{n=1}^{\infty}\displaystyle\sum_{m=2}^{\infty}|a_m|\frac{1}{n^m}\leq\displaystyle\sum_{n=1}^{\infty}\displaystyle\sum_{m=2}^{\infty}|a_m|\frac{1}{n^2}<\infty$

Hence, by Cauchy’s double series theorem, we can switch the order of summation:

$\displaystyle{\sum_{n=1}^{\infty}f\left(\frac{1}{n}\right)=\sum_{n=1}^{\infty}\sum_{m=2}^{\infty}a_m\frac{1}{n^m}=\sum_{m=2}^{\infty}a_m\sum_{n=1}^{\infty}\frac{1}{n^m}=\sum_{n=2}^{\infty}a_n\zeta(n)}$

This theorem implies that

$\displaystyle{\frac{\pi}{4}=\sum_{n=1}^{\infty}f\left(\frac{1}{n}\right)=\frac{1}{4}\sum_{n=2}^{\infty}\frac{3^{n-1}-1}{4^{n-1}}\zeta(n)=\frac{1}{4}\sum_{n=1}^{\infty}\frac{3^{n}-1}{4^{n}}\zeta(n+1)}$