## Posts Tagged 'Analysis'

### Enigmas of Infinity

Children sometimes amuse themselves searching for the biggest number. After trying millions, billions and trillions, they realize that there is no end to the game: however big a number may be, we can always add 1 to produce a bigger number: the set of counting numbers is infinite. The concept of infinity has intrigued philosophers since antiquity, and it leads to many surprises and paradoxical results [TM110 or search for “thatsmaths” at irishtimes.com].

### Taylor Expansions from India

FIg. 1: Brook Taylor (1685-1731). Image from NPG.

The English mathematician Brook Taylor (1685-1731) introduced the calculus of finite differences in his Methodus Incrementorum Directa et Inversa, published in 1715. This work contained the famous formula known today as Taylor’s formula. In 1772, Lagrange described it as “the main foundation of differential calculus” (Wikipedia: Brook Taylor). Taylor also wrote a treatise on linear perspective (see Fig. 1).

It is noteworthy that the series for ${\sin x}$, ${\cos x}$ and ${\arctan x}$ were known to mathematicians in India about 400 years before Taylor’s time.
Continue reading ‘Taylor Expansions from India’

### Which is larger, e^pi or pi^e?

Which is greater, ${x^y}$ or ${y^x}$? Of course, it depends on the values of x and y. We might consider a particular case: Is ${e^\pi > \pi^e}$ or ${\pi^e > e^\pi}$?

Contour plot of x^y – y^x, positive in the yellow regions, negative in the blue ones.

### Slicing Doughnuts

It is well-known that an ellipse is the locus of all points such that the sum of their distances from two fixed points, the foci, is constant. Thus, a gardener may map out an elliptical flower-bed by driving two stakes into the ground, looping a rope around them and pulling it taut with a pointed stick, tracing out a curve on the ground.

### Random Harmonic Series

We consider the convergence of the random harmonic series

$\displaystyle R = \sum_{n=1}^{\infty}\frac{\sigma_{n}}{n}$

where ${\sigma_n\in\{-1,+1\}}$ is chosen randomly with probability ${1/2}$ of being either plus one or minus one. It follows from the Kolmogorov three-series theorem that the series is “almost surely” convergent.

### Sigmoid Functions: Gudermannian and Gompertz Curves

The Gudermannian is named after Christoph Gudermann (1798–1852). The Gompertz function is named after Benjamin Gompertz (1779–1865). These are two amongst several sigmoid functions. Sigmoid functions find applications in many areas, including population dynamics, artificial neural networks, cartography, control systems and probability theory. We will look at several examples in this class of functions.

### The Power Tower Fractal

We can construct a beautiful fractal set by defining an operation of iterating exponentials and applying it to the numbers in the complex plane. The operation is tetration and the fractal is called the power tower fractal or sometimes the tetration fractal. A detail of the set is shown in the figure here.

Detail of the power tower fractal.