Wheat and chessboard problem

The problem appears in different stories about the invention of chess. One of them includes the geometric progression problem. The story is first known to have been recorded in 1256 by Ibn Khallikan.^[4] Another version has the inventor of chess (in some tellings Sessa, an ancient Indian Minister) request his ruler give him wheat according to the wheat and chessboard problem. The ruler laughs it off as a meager prize for a brilliant invention, only to have court treasurers report the unexpectedly huge number of wheat grains would outstrip the ruler's resources. Versions differ as to whether the inventor becomes a high-ranking advisor or is executed.^[5]

Macdonnell also investigates the earlier development of the theme.^[6]

[According to al-Masudi's early history of India], shatranj, or chess was invented under an Indian king, who expressed his preference for this game over backgammon. [...] The Indians, he adds, also calculated an arithmetical progression with the squares of the chessboard. [...] The early fondness of the Indians for enormous calculations is well known to students of their mathematics, and is exemplified in the writings of the great astronomer Āryabaṭha (born 476 A.D.). [...] An additional argument for the Indian origin of this calculation is supplied by the Arabic name for the square of the chessboard, (بيت, "beit"), 'house'. [...] For this has doubtless a historical connection with its Indian designation koṣṭhāgāra, 'store-house', 'granary' [...].

 

The simple, brute-force solution is just to manually double and add each step of the series:

${\displaystyle T_{64}}$  = 1 + 2 + 4 + ..... + 9,223,372,036,854,775,808 = 18,446,744,073,709,551,615

where ${\displaystyle T_{64}}$  is the total number of grains.

The series may be expressed using exponents:

${\displaystyle T_{64}=2^{0}+2^{1}+2^{2}+\cdots +2^{63}}$ 

and, represented with capital-sigma notation as:

${\displaystyle \sum _{k=0}^{63}2^{k}.\,}$ 

It can also be solved much more easily using:

${\displaystyle T_{64}=2^{64}-1.\,}$ 

A proof of which is:

${\displaystyle s=2^{0}+2^{1}+2^{2}+\cdots +2^{63}.}$ 

Multiply each side by 2:

${\displaystyle 2s=2^{1}+2^{2}+2^{3}+\cdots +2^{63}+2^{64}.}$ 

Subtract original series from each side:

${\displaystyle {\begin{aligned}2s-s&=\qquad \quad {\cancel {2^{1}}}+{\cancel {2^{2}}}+\cdots +{\cancel {2^{63}}}+2^{64}\\&\quad -2^{0}-{\cancel {2^{1}}}-{\cancel {2^{2}}}-\cdots -{\cancel {2^{63}}}\\&=2^{64}-2^{0}\\\therefore s&=2^{64}-1.\end{aligned}}}$ 

The solution above is a particular case of the sum of a geometric series, given by

${\displaystyle a+ar+ar^{2}+ar^{3}+\cdots +ar^{n-1}=\sum _{k=0}^{n-1}ar^{k}=a\,{\frac {1-r^{n}}{1-r}},}$ 

where ${\displaystyle a}$  is the first term of the series, ${\displaystyle r}$  is the common ratio and ${\displaystyle n}$  is the number of terms.

In this problem ${\displaystyle a=1}$ , ${\displaystyle r=2}$  and ${\displaystyle n=64}$ .

Thus,

${\displaystyle {\begin{aligned}\sum _{k=0}^{n-1}2^{k}&=2^{0}+2^{1}+2^{2}+\cdots +2^{n-1}\\&=2^{n}-1\end{aligned}}}$ 

for ${\displaystyle n}$  being any positive integer.

The exercise of working through this problem may be used to explain and demonstrate exponents and the quick growth of exponential and geometric sequences. It can also be used to illustrate sigma notation. When expressed as exponents, the geometric series is: 2⁰ + 2¹ + 2²  + 2³ + ... and so forth, up to 2⁶³. The base of each exponentiation, "2", expresses the doubling at each square, while the exponents represent the position of each square (0 for the first square, 1 for the second, and so on.).

The number of grains is the 64th Mersenne number.

 

In technology strategy, the "second half of the chessboard" is a phrase, coined by Ray Kurzweil,^[7] in reference to the point where an exponentially growing factor begins to have a significant economic impact on an organization's overall business strategy. While the number of grains on the first half of the chessboard is large, the amount on the second half is vastly (2³² > 4 billion times) larger.

The number of grains of wheat on the first half of the chessboard is 1 + 2 + 4 + 8 + ... + 2,147,483,648, for a total of 4,294,967,295 (2³² − 1) grains, or about 279 tonnes of wheat (assuming 65 mg as the mass of one grain of wheat).^[8]

The number of grains of wheat on the second half of the chessboard is 2³² + 2³³ + 2³⁴ + ... + 2⁶³, for a total of 2⁶⁴ − 2³² grains. This is equal to the square of the number of grains on the first half of the board, plus itself. The first square of the second half alone contains one more grain than the entire first half. On the 64th square of the chessboard alone, there would be 2⁶³ = 9,223,372,036,854,775,808 grains, more than two billion times as many as on the first half of the chessboard.

On the entire chessboard there would be 2⁶⁴ − 1 = 18,446,744,073,709,551,615 grains of wheat, weighing about 1,199,000,000,000 metric tons. This is over 1,600 times the global production of wheat (729 million metric tons in 2014 and 780.8 million tonnes in 2019).^[9]

Carl Sagan titled the second chapter of his final book "The Persian Chessboard" and wrote, referring to bacteria, that "Exponentials can't go on forever, because they will gobble up everything."^[10] Similarly, The Limits to Growth uses the story to present suggested consequences of exponential growth: "Exponential growth never can go on very long in a finite space with finite resources."^[11]

• Legend of the Ambalappuzha Paal Payasam
• Malthusian growth model
• Moore's law
• Orders of magnitude (data)
• Technology strategy
• The Limits to Growth

 

Look up wheat and chessboard problem in Wiktionary, the free dictionary.

• Weisstein, Eric W. "Wheat and Chessboard Problem". MathWorld.
• Salt and chessboard problem - A variation on the wheat and chessboard problem with measurements of each square.
•   Learning materials related to Math Adventures/Wheat and the Chessboard at Wikiversity