`The Annunciation', Leonardo da Vinci

Since the days of old, artists as well a geometricians have known that there is a special, aesthetically pleasing rectangle with width 1, length x and the following property: When a square of side 1 is removed, the rectangle that remains has the same proportions as the original rectangle. This ratio is known as the golden section.

The origin of the golden section dates to the early 11th century. In 1202, the mathematician Leonardo of Pisa, also called Fibonacci, published an influential treatise, Liber abaci. It contained the following recreational problem: "How many pairs of rabbits can be produced from a single pair in one year if it is assumed that every month each pair begets a new pair which from the second month becomes productive?" Straightforward calculation generates the following sequence:

The second row represents the first 12 terms of the sequence now known by Fibonacci's name, in which each term (except the first two) is found by adding the two terms immediately preceding; in general, x_n = x_{n - 1} + x_{n - 2}, a relation that was not recognized until about 1600.

Over the years, especially in the middle decades of the 20th century, the properties of the Fibonacci numbers have been extensively studied, resulting in a considerable literature. Their properties seem inexhaustible; for example, x_n+1 x_n-1 = x_n² + (-1)ⁿ. Another formula for generating the Fibonacci numbers is attributed to Édouard Lucas:

The ratio ({sqroot 5} + 1) : 2 = 1.618 . . . , designated as , is known as the golden number; the ratio ({sqroot 5} - 1) : 2, the reciprocal of , is equal to 0.618 . . . . Both these ratios are related to the roots of x² - x - 1 = 0, an equation derived from the Divine Proportion of the 15th-century Italian mathematician Luca Pacioli, namely, a/b = b/(a + b), when a < b, by setting x = b/a. In short, dividing a segment into two parts in mean and extreme proportion, so that the smaller part is to the larger part as the larger is to the entire segment, yields the so-called Golden Section, an important concept in both ancient and modern artistic and architectural design. Thus, a rectangle the sides of which are in the approximate ratio of 3 : 5 ( ^-1 = 0.618 . . .), or 8 : 5 ( = 1.618 . . .), is presumed to have the most pleasing proportions, aesthetically speaking.

Raising the golden number to successive powers generates the sequence that begins as follows:

In this sequence the successive coefficients of the radical {sqroot 5} are Fibonacci's 1, 1, 2, 3, 5, 8, while the successive second terms within the parentheses are the so-called Lucas sequence: 1, 3, 4, 7, 11, 18. The Lucas sequence shares the recursive relation of the Fibonacci sequence; that is, x_n = x_n-1 + x_n-2.

If a golden rectangle ABCD is drawn and a square ABEF is removed, the remaining rectangle ECDF is also a golden rectangle. If this process is continued and circular arcs are drawn, the curve formed approximates the logarithmic spiral, a form found in nature (see Figure above). The logarithmic spiral is the graph of the equation r = k, in polar coordinates, where k = ^2/. The Fibonacci numbers are also exemplified by the botanical phenomenon known as phyllotaxis. Thus, the arrangement of the whorls on a pinecone or pineapple, of petals on a sunflower, and of branches from some stems follows a sequence of Fibonacci numbers or the series of fractions

Excerpt from the Encyclopedia Britannica without permission.