Π and E, and the Most Beautiful Theorem in Mathematics

π and e, and the most beautiful theorem in mathematics

Robin Wilson Leonhard Euler (1707-1783) The ‘circle number’ π

22 π = 3.14159… (< /7) is

• the ratio of the circumference C of a circle to its diameter d:

π = C/d, so C = π d = 2 π r (r = radius)

• the ratio of the area A of a circle to the square of its radius r: 2 2 2 π = A/r , so A = π r = π d /4 π to 500 decimal places

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Sato Moshun’s Tengen Shinan, 1698 Mesopotamian value of π (c.1800 BC)

Ratio of the perimeter of the hexagon to the circumference of the circle is 0; 57, 36

6r 3 57 36 /2πr = /π = /60 + /3600 1 So π = 3 /8 = 3.125 An Egyptian problem in geometry Problem 50. Example of a round field of diameter 9 khet. What is its area?

Answer: 1 Take away /9 of the diameter, namely 1; the remainder is 8. Multiply 8 times 8; it makes 64. So it contains 64 setat of land. Area = (d – 1/ d)2 = (8/ d)2 = 256/ r2 9 9 81 which is about 3.160 r2 Using polygons (Antiphon & Bryson)

Antiphon: π > 2 π > 2.828

Bryson: π < 4 π < 3.32 Archimedes’ value of π perimeter/area of inside 6-gon < circumference/area of circle < perimeter/area of outside 6-gon

Now double the number of sides: 6, 12, 24, 48, 96

10 1 Result: 3 /71 < π < 3 /7 3.14084 3.14286 Chinese values for π Zhang Heng (AD 100) π = √10 (≈3.162)

← Liu Hui (AD 263) π = 3.14159 (3072 sides)

Zu Chongzhi (AD 500) π = 3.1415926 (24,576 sides)

and π = 355/ 113 More polygons

Fibonacci (Italy, 1220) π = 3.141818 (96 sides)

Al-Kashi (Samarkand, 1424) 16 d. p. (3 × 228 sides)

François Viète (French, 1579) 9 d. p. (393216 sides) 355 Valentin Otho (Germany, 1573) π = /113 Adriaan Anthonisz (Dutch, 1585) 333 377 355 9 /106 < π < /120 and /113 (10 sides)

Adriaan van Roomen (Dutch, 1593) 15 d. p. (230 sides)

Ludolph van Ceulen (Dutch) (1540-1610)

1596: 20 d. p. 515,396,075,520 = 60 × 233 sides →

1610: 35 d. p. 4,611,686,018,427,387,904 = 262 sides François Viète & John Wallis 1579: Viète 2/π = cos π/4 × cos π/8 × cos π/16 × cos π /32 × . . . = ½ √2 × ½ √(2 + √2) × ½ √(2 + √(2 + √2)) × ½ √(2 + √(2 + √(2 + √2))) × . . . 1656: 4 = 3 × 3 × 5 × 5 × 7 × 7 × 9 × 9 × . . . Wallis π 2 × 4 × 4 × 6 × 6 × 8 × 8 × 10 × . . . The tan−1 (or arctan) function tan θ = a/b θ = tan-1 (a/b)

tan π/4 = 1, so tan-1 (1) = π/4 tan π/6 = 1/√3, so tan-1 (1/√3) = π/6

-1 1 -1 1 tan ( /2) + tan ( /3) = π/4

In general:

−1 −1 −1  x + y  tan x + tan y = tan    1 − xy  The series for tan−1 x

–1 1 3 1 5 1 7 tan x = x – /3 x + /5 x – /7 x + . . . So, with x = 1 we have: 1 1 1 π/4 = 1 – /3 + /5 – /7 + . . . But this converges extremely slowly: 300 terms give only two decimal places of π. Much better is: –1 1 –1 1 π/4 = tan ( /2) + tan ( /3) 1 1 1 3 1 1 5 1 1 7 = { /2 – /3 ( /2) + /5 ( /2) – /7 ( /2) + . . . } 1 1 1 3 1 1 5 1 1 7 – { /3 – /3 ( /3) + /5 ( /3) – /7 ( /3) + . . .} , which converges much faster. Machin’s tan–1 formula (1706)

–1 1 –1 1 π = 16 tan ( /5) – 4 tan ( /239) 1 1 1 3 1 1 5 1 1 7 = 16 { /5 – /3 ( /5) + /5 ( /5) – /7 ( /7) + . . . } 1 1 1 3 1 5 1 1 7 – 4 {( /239) – /3 ( /239) + ( /239) – /7 ( /239) + . . .} Machin used this series to calculate π to 100 decimal places

John Machin was Gresham Professor of Astronomy from 1713 to 1751 William Jones introduces π (1706) A new value for π Other formulas included one of Leonhard Euler (1755): –1 1 –1 3 π = 20 tan ( /7) + 8 tan ( /79).

This was later used by Jurij Vega to find π to 136 decimal places, for many years the best value known. But persistent rumours pointed to an earlier value correct to 152 decimal places, hidden in Oxford’s Bodleian library. This remained undiscovered until 2014 when Benjamin Wardhaugh found it an an obscure book of 1721. It used 314 terms of the tan–1 series in the form –1 1 π = 6 tan ( /√3) 1 1 1 1 2 1 1 3 = √12 {1 – /3 ( /3) + /5 ( /3) – /7 ( /3) + . . . } William Shanks’s value of π In 1873, using Machin’s formula, William Shanks calculated π to 707 decimal places, which can be viewed in the Palais de la Découverte in Paris:

Unfortunately, . . . Some weird results

Ramanujan (1914):

The Chudnovsky brothers (1989):

Bailey, Borwein and Plouffe (1995): Enter the computer 1949 ENIAC (Maryland) 2037 70 hours

1955 NORC 3089 13 minutes

1957 PEGASUS (England) 10,021 33 hours

1958 IBM704 (Paris) 10,000 100 minutes

1961 IBM7090 (London) 20,000 39 minutes

1961 Shanks/Wrench (New York) 100,265 8.7 hours

1973 CDC 7600 1 million 23.3 hours

1989 Chudnovskys (New York) 1 billion

2002 Kanada 1 trillion

2011 Shigeni Kondo etc. 10 trillion . . . Circling the earth The earth’s circumference is about 25,000 miles (132 million feet)

Tie a string tightly around the earth. Then extend the string by just 2π (≈ 6.3) feet, and prop it up equally all around the earth.

How high above the ground is the string? Circling the earth

The earth’s circumference is about 25,000 miles. Tie a string tightly around the earth, extend the string by 2π (≈ 6.3) feet, and prop it up equally all around the earth. How high above the ground is the string?

If the earth’s radius is r, then the original string has length 2πr . When we extend the string by 2π , the new circumference is 2πr + 2π = 2π (r + 1) , so the new radius is r + 1 – so everywhere the string is now one foot off the ground. The ‘exponential number’ e e = 2.7182818284590452360287471352…

The letter e was first used in an unpublished paper of Euler around 1727, and in a letter of 1731. It first appeared in print in 1736 in Volume 1 of his Mechanica: ‘where e denotes the number whose hyperbolic ↑ logarithm is 1’. A chessboard problem

The wealthy king of a certain country was so impressed by the new game of chess that he offered the wise man who invented it any reward he wished.

The wise man replied: ‘My prize is for you to give me 1 grain of wheat for the first square of the chessboard, 2 grains for the second square, 4 grains for the third square, and so on, doubling the number of grains on each successive square until the chessboard is filled.’ A chessboard problem

Total number of grains: 1 + 2 + 22 + 23 + . . . + 263 = 264 − 1 = 18,446,744,073,709,551,615 – enough wheat to form a pile the size of Mount Everest. Exponential growth polynomial growth exponential growth n : 1, 2, 3, 4, 5, . . . 2n : 1, 2, 4, 8, 16, 32, . . . 2 n : 1, 4, 9, 16, 25, . . . n n3 : 1, 8, 27, 64, 125, . . . 3 : 1, 3, 9, 27, 81, 243, . . .

n = 10 n = 30 n = 50 polynomial n 0.00001 seconds 0.00003 seconds 0.00005 seconds n2 0.0001 seconds 0.0009 seconds 0.0025 seconds n3 0.001 seconds 0.027 seconds 0.125 seconds n5 0.1 seconds 24.3 seconds 5.2 minutes exponential 2n 0.001 seconds 17.9 minutes 35.7 years 3n 0.059 seconds 6.5 years 2.3 × 1010 years

In the long run, exponential growth usually exceeds polynomial growth. An interest-ing problem

Jakob Bernoulli asked: How much is earned when compound interest is calculated yearly? twice a year? . . . , every day? n times a year? . . . , continuously? Invest £1 at 100% interest per year: 1 1 2 twice a year: £1 → £(1 + /2) = £1.50 → £(1 + /2) = £2.25 1 1 2 1 n n times a year: £1 → £(1 + /n) → £ (1 + /n) → . . . → £ (1 + /n)

As n increases, these numbers tend to the limiting value of e. Leonhard Euler (1707-1783) Some properties of e

n • e = limn (1 + 1/n) →∞ x n • e = limn→∞ (1 + x/n)

1 1 1 1 • e = 1 + /1! + /2! + /3! + /4! + . . .

• ex = 1 + x/1! + x2/2! + x3/3! + . . .

• The slope of the curve y = ex at the value x is y = ex : → dy/dx = y John Napier’s logarithms (1614) Aim: to turn multiplication and division problems into addition and subtraction ones – that is, to turn geometric progressions into arithmetic ones.

log (a × b) = log a + log b – log 1 log 1 = 161,180,956 Henry Briggs’s logarithms (1617) [Napier has] set my Head and hands a Work with his new and remarkable logarithms . . . I never saw a book which pleased me better or made me more wonder. I myself, when expounding this doctrine to my auditors at Gresham College, remarked that it would be much more convenient that 0 should be kept for the logarithm of the whole sine [log 1 = 0]

Briggs introduced ‘logs to base 10’, which became widely used (for example, by mariners) and for which: log (a × b) = log a + log b exp and log are inverse to each other

If y = ex, then x = log y so log ex = x and elog y = y

So the exponential law ex × ey = ex+y and the logarithm law log (x × y) = log x + log y are essentially the same result. Derangements Given any n letters a, b, c, d, e, . . . , in how many ways can we rearrange them so that none is in its original position? n = 4: badc, bcda, bdac, cadb, cdab, cdba, dabc, dcab, dcba

n 1 2 3 4 5 6 7 8 Dn 0 1 2 9 44 265 1854 14,833

De Moivre/Euler:

Dn = n! {1 – 1/1! + 1/2! – 1/3! + . . . ± 1/n!}, which is very close to n! × e–1 = n!/e.

For example, when n = 8, n!/e = 14,832.9 and Dn = 14,833. Exponential growth as predicted by Thomas Malthus (1796)

If N(t), the size of a population at time t, grows at a fixed rate k proportional to its size, then dN/dt = kN – or dN/N = k dt . This can be integrated to give kt log N = kt + const. – or N = N0 e , where N0 is the initial population.

Similarly for the decay of radium, the cooling of a cup of tea, etc. Euler’s equation – ‘the most beautiful theorem in mathematics’

Five important constants: 1 – the counting number iπ 0 – the nothingness number e + 1 = 0 π – the circle number (or eiπ = –1) e – the exponential number i – the imaginary number

• Feynman (aged 14): ‘the most remarkable formula in math’ • M. Atiyah: ‘the mathematical equivalent of Hamlet’s To be or not to be: very succinct, but at the same time very deep’ • Featured in The Simpsons and in a criminal court case A near-miss: Johann Bernoulli What is log (–1)? 2 log (–1) = log (–1) + log (–1) = log (–1 × –1) = log (1) = 0, so log (–1) = 0 ; similarly, log (–x) = log x, for all x.

Bernoulli also found the area A of the sector on the right to be A = (a2/4i) × log (x + iy)/(x – iy). Euler then put x = 0: A = (a2/4i) × log (–1) = πa2/4 . So log (–1) ≠ 0, and we deduce log (–1) = iπ Taking exponentials then gives Euler’s equation in the form eiπ = –1 Euler’s identity There’s no real reason why there should be any connection between ex and cos x and sin x.

ex = 1 + x/1! + x2/2! + x3/3! + x4/4! + x5/5! + . . . cos x = 1 – x2/2! + x4/4! – . . . sin x = x – x3/3! + x5/5! – . . . so eix = 1 + ix/1! + (ix)2/2! + (ix)3/3! + (ix)4/4! + (ix)5/5! + . . . = (1 – x2/2! + x4/4! – . . .) + i (x – x3/3! + x5/5! – . . . ) = cos x + i sin x

eix = cos x + i sin x Euler’s identity: eiv = cos v + i sin v Two postage stamps Another near-miss: Roger Cotes Roger Cotes introduced radian measure for angles, and worked with Newton on the Principia Mathematica.

He also investigated the surface area of an ellipsoid, and found two expressions for it involving logarithms and trigonometry, and both involving an angle φ.

Equating these, he found that log (cos φ + i sin φ) = iφ. Taking exponentials then gives Euler’s identity Consequences of Euler’s identity eix = cos x + i sin x

1. Put x = π (= 180°): then eiπ = cos π + i sin π = – 1 + 0 i = – 1, which is Euler’s equation. 2. For any number n, (cos x + i sin x)n = (eix)n = ei (nx) = cos nx + i sin nx : this is De Moivre’s theorem. 3. Since eix = cos x + i sin x and e–ix = cos x – i sin x, cos x = (eix + e–ix)/2 and sin x = (eix – e–ix)/2i . Who discovered ‘Euler’s equation’? Bernoulli/Euler: iπ = log (–1)

Roger Cotes: log (cos φ + i sin φ) = iφ

Euler seems never to have written down eiπ + 1 = 0 explicitly – though he surely realised that it follows from his identity, eix = cos x + i sin x .

We don’t know who first stated it explicitly – though there’s an early appearance in 1813-14.

Most people attribute it to Euler, to honour this great mathematical pioneer. Who discovered ‘Euler’s equation’? Bernoulli/Euler: iπ = log (–1)

Roger Cotes: log (cos φ + i sin φ) = iφ

Euler seems never to have written down eiπ + 1 = 0 explicitly – though he surely realised that it follows from his identity, eix = cos x + i sin x .

We don’t know who first stated it explicitly – though there’s an early appearance in 1813-14.

Most people attribute it to Euler, to honour this great mathematical pi-0ne-e-r. Euler’s pioneering equation (Oxford University Press)

Gresham College book launch at Barnard’s Inn Hall on 15 February 2018 at 6 pm