# Tag Archives: Numberphile

# the Riemann Hypothesis

The **Riemann zeta function** or **Euler–Riemann zeta function**, *ζ*(*s*), is a function of a complex variable *s* that analytically continues the sum of the infinite series

which converges when the real part of *s* is greater than 1. More general representations of *ζ*(*s*) for all *s* are given below. The Riemann zeta function plays a pivotal role in analytic number theory and has applications in physics, probability theory, and applied statistics.

As a function of a real variable, Leonhard Euler first introduced and studied it in the first half of the eighteenth century without using complex analysis, which was not available at the time. Bernhard Riemann‘s 1859 article “On the Number of Primes Less Than a Given Magnitude” extended the Euler definition to a complex variable, proved its meromorphic continuation andfunctional equation, and established a relation between its zeros and the distribution of prime numbers.^{[2]}

The values of the Riemann zeta function at even positive integers were computed by Euler. The first of them, *ζ*(2), provides a solution to the Basel problem. In 1979 Apéry proved the irrationality of *ζ*(3). The values at negative integer points, also found by Euler, are rational numbers and play an important role in the theory of modular forms. Many generalizations of the Riemann zeta function, such as Dirichlet series, Dirichlet L-functions and L-functions, are known.

In mathematics, the **Riemann hypothesis** is a conjecture that the Riemann zeta function has its zeros only at the negative even integers and the complex numbers with real part 1/2. It was proposed by Bernhard Riemann (1859), after whom it is named. The name is also used for some closely related analogues, such as the Riemann hypothesis for curves over finite fields.

The Riemann hypothesis implies results about the distribution of prime numbers. Along with suitable generalizations, some mathematicians consider it the most important unresolved problem in pure mathematics (Bombieri 2000). The Riemann hypothesis, along with Goldbach’s conjecture, is part of Hilbert’s eighth problem in David Hilbert‘s list of 23 unsolved problems; it is also one of the Clay Mathematics Institute‘s Millennium Prize Problems.

The Riemann zeta function ζ(*s*) is a function whose argument *s* may be any complex number other than 1, and whose values are also complex. It has zeros at the negative even integers; that is, ζ(*s*) = 0 when *s* is one of −2, −4, −6, …. These are called its **trivial zeros**. However, the negative even integers are not the only values for which the zeta function is zero. The other ones are called *non-trivial zeros*. The Riemann hypothesis is concerned with the locations of these non-trivial zeros, and states that:

- The real part of every non-trivial zero of the Riemann zeta function is 1/2.

Thus, if the hypothesis is correct, all the non-trivial zeros lie on the **critical line** consisting of the complex numbers 1/2 + *i t*, where *t* is areal number and *i* is the imaginary unit.

There are several nontechnical books on the Riemann hypothesis, such as Derbyshire (2003), Rockmore (2005), (Sabbagh 2003a,2003b), du Sautoy (2003). The books Edwards (1974), Patterson (1988), Borwein et al. (2008) and Mazur & Stein (2015) give mathematical introductions, while Titchmarsh (1986), Ivić (1985) and Karatsuba & Voronin (1992) are advanced monographs.

# Divergent series

In mathematics, a series or integral is said to be **conditionally convergent** if it converges, but it does not converge absolutely.

## Definition

More precisely, a series is said to **converge conditionally** if exists and is a finite number (not ∞ or −∞), but

A classic example is the alternating series given by

which converges to , but is not absolutely convergent (see Harmonic series).

Bernhard Riemann proved that a conditionally convergent series may be rearranged to converge to any sum at all, including ∞ or −∞; see *Riemann series theorem*.

A typical conditionally convergent integral is that on the non-negative real axis of (see Fresnel integral).

**Published on Apr 22, 2016**

The Mathologer sets out to make sense of 1+2+3+ … = -1/12 and some of those other notorious, crazy-looking infinite sum identities. The starting point for this video is the famous letter that led to the discovery of self-taught mathematical genius Srinivasa Ramanujan in 1913 (Ramanujan is the subject of the movie “The man who knew infinity” that just started showing in cinemas.) Find out about how these identities come up in Ramanujan’s work, the role of “just do it” in math, the rules for adding infinite sums on Earth and other worlds, and what all this has to do with the mathematical super star the Riemann Zeta function.

If you want to watch some other videos that deal with these strange identities I recommend the following:

https://youtu.be/0Oazb7IWzbA (a Numberphile video featuring the mathematician Edward Frenkel)

https://youtu.be/XFDM1ip5HdU

Burkard

P.S.: If you know calculus and want to read up on all this some more, beyond what is readily available via the relevant Wiki pages and other internet resources, I recommend you read the last chapter of the book by Konrad Knopp, Theory and applications of infinite series, Dover books, 1990 (actually if you know German, read the extended version of this chapter in the original 1924 edition of the book).

Prof Edward Frenkel’s book Love and Math: http://amzn.to/1g6XP6j

Professor Frenkel is a mathematics professor at the University of California, Berkeley – http://edwardfrenkel.com

The Millennium Prize at the Clay Mathematics Institute: http://www.claymath.org

Number Line: http://youtu.be/JmyLeESQWGw

# Taxi cab numbers

# Fermat’s Last Theorem near misses?

In mathematics, the *n*th **taxicab number**, typically denoted Ta(*n*) or Taxicab(*n*), is defined as the smallest number that can be expressed as a sum of two *positive* algebraic cubesin *n* distinct ways. The concept was first mentioned in 1657 by Bernard Frénicle de Bessy, and was made famous in the early 20th century by a story involving Srinivasa Ramanujan. In 1938, G. H. Hardy and E. M. Wright proved that such numbers exist for all positive integers *n*, and their proof is easily converted into a program to generate such numbers. However, the proof makes no claims at all about whether the thus-generated numbers are *the smallest possible* and thus it cannot be used to find the actual value of Ta(*n*).

The restriction of the summands to positive numbers is necessary, because allowing negative numbers allows for more (and smaller) instances of numbers that can be expressed as sums of cubes in *n* distinct ways. The concept of a cabtaxi number has been introduced to allow for alternative, less restrictive definitions of this nature. In a sense, the specification of two summands and powers of three is also restrictive; a generalized taxicab number allows for these values to be other than two and three, respectively.

Ta(2), also known as the **Hardy–Ramanujan number**, was first published by Bernard Frénicle de Bessy in 1657 and later immortalized by an incident involving mathematicians G. H. Hardy and Srinivasa Ramanujan. As told by Hardy [1]:

“ | I remember once going to see him when he was lying ill at Putney. I had ridden in taxi-cab No. 1729, and remarked that the number seemed to be rather a dull one, and that I hoped it was not an unfavourable omen. “No”, he replied, “it is a very interesting number; it is the smallest number expressible as the sum of two [positive] cubes in two different ways.” | ” |

The subsequent taxicab numbers were found with the help of supercomputers. John Leech obtained Ta(3) in 1957. E. Rosenstiel, J. A. Dardis and C. R. Rosenstiel found Ta(4) in 1991. J. A. Dardis found Ta(5) in 1994 and it was confirmed by David W. Wilson in 1999.^{[1]}^{[2]} Ta(6) was announced by Uwe Hollerbach on the NMBRTHRY mailing list on March 9, 2008,^{[3]} following a 2003 paper by Calude et al. that gave a 99% probability that the number was actually Ta(6).^{[4]} Upper bounds for Ta(7) to Ta(12) were found by Christian Boyer in 2006.^{[5]}