If a chessboard were to have wheat placed upon each square such that one grain were placed on the first square, two on the second, four on the third, and so on (doubling the number of grains on each subsequent square), how many grains of wheat would be on the chessboard at the finish?
The problem may be solved using simple addition. With 64 squares on a chessboard, if the number of grains doubles on successive squares, then the sum of grains on all 64 squares is: 1 + 2 + 4 + 8 + ... and so forth for the 64 squares. The total number of grains equals 18,446,744,073,709,551,615 (the 64th Mersenne number), much higher than what most intuitively expect.
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, etc.).
The simple, brute-force solution is to just manually double and add each step of the series:
The series may be expressed using exponents:
and, represented with capital-sigma notation as:
It can also be solved much more easily using:
A proof of which is:
Multiply each side by 2:
Subtract original series from each side:
The solution above is a particular case of the sum of a geometric series, given by
where is the first term of the series, is the common ratio and is the number of terms.
In this problem , and .
The wheat and chess 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. 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.
Macdonnell also investigates the earlier development of the theme.
This exercise can be used to demonstrate how quickly exponential sequences grow, as well as to introduce exponents, zero power, capital-sigma notation and geometric series. Updated for modern times using pennies and the hypothetical question, "Would you rather have a million dollars or the sum of a penny doubled every day for a month?", the formula has been used to explain compounded interest.
In technology strategy, the "second half of the chessboard" is a phrase, coined by Ray Kurzweil, 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 , 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).
The number of grains of wheat on the second half of the chessboard is , 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 more grains 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 about 1,645 times the global production of wheat in 2014 (729,000,000 metric tons).
Carl Sagan titled the second chapter of his final book The Persian Chessboard and wrote that when referring to bacteria, "Exponentials can't go on forever, because they will gobble up everything." 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."
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