archimedes' cattle problem

Around 2300 years ago, Archimedes developed a “BigInteger” scheme in order to express integers larger than supported by the Greek numeral system, ie, larger than a myriad-myriad = $ 10^8 $. His system is described in a paper he wrote called The Sand Reckoner, where he calculated an upper bound of $ 10^{63} $ for the number of grains of sand that could fit in the known universe.

It was around this time that Archimedes sent a letter to a colleague in Egypt with a problem that would surely strain any BigInteger system.

the letter

If thou art diligent and wise, O stranger, compute the number of cattle of the Sun, who once upon a time grazed on the fields of the Thrinacian isle of Sicily, divided into four herds of different colours, one milk white, another a glossy black, a third yellow and the last dappled. In each herd were bulls, mighty in number according to these proportions: Understand, stranger, that the white bulls were equal to a half and a third of the black together with the whole of the yellow, while the black were equal to the fourth part of the dappled and a fifth, together with, once more, the whole of the yellow. Observe further that the remaining bulls, the dappled, were equal to a sixth part of the white and a seventh, together with all of the yellow. These were the proportions of the cows: The white were precisely equal to the third part and a fourth of the whole herd of the black; while the black were equal to the fourth part once more of the dappled and with it a fifth part, when all, including the bulls, went to pasture together. Now the dappled in four parts were equal in number to a fifth part and a sixth of the yellow herd. Finally the yellow were in number equal to a sixth part and a seventh of the white herd. If thou canst accurately tell, O stranger, the number of cattle of the Sun, giving separately the number of well-fed bulls and again the number of females according to each colour, thou wouldst not be called unskilled or ignorant of numbers, but not yet shalt thou be numbered among the wise.

But come, understand also all these conditions regarding the cattle of the Sun. When the white bulls mingled their number with the black, they stood firm, equal in depth and breadth, and the plains of Thrinacia, stretching far in all ways, were filled with their multitude. Again, when the yellow and the dappled bulls were gathered into one herd they stood in such a manner that their number, beginning from one, grew slowly greater till it completed a triangular figure, there being no bulls of other colours in their midst nor none of them lacking. If thou art able, O stranger, to find out all these things and gather them together in your mind, giving all the relations, thou shalt depart crowned with glory and knowing that thou hast been adjudged perfect in this species of wisdom.

the cattle problem

Translating the letter, there are four colors of cattle: black, white, yellow, and dappled. We label the bulls by the lower-case letters $ b $, $ w $, $ y $, $ d $ and the cows by the upper-case letters $ B $, $ W $, $ Y $, $ D $. The problem is given in two parts.

[
  [  6,  -5,   0,  -6,   0,   0,   0,   0],
  [  0,  20,  -9, -20,   0,   0,   0    0],
  [-13,   0,  42, -42,   0,   0,   0,   0],
  [  0,  -7,   0,   0,  12,  -7,   0,   0],
  [  0,   0,  -9,   0,   0,  20,  -9,   0],
  [  0,   0,   0, -11,   0,   0,  30, -11],
  [-13,   0,   0,   0, -13,   0,   0,  42],
]

continued fractions

The continued fraction algorithm begins with a nonzero real number $ \alpha $, computes its integer part called the quotient $ q $, and its fractional part $ \beta = \alpha − q $. This $ beta $ is between $ 0 $ and $ 1 $, so it’s reciprocal is greater than 1.

At each iteration of the algorithm, $ \alpha $ is replaced by the reciprocal of $ \beta $ and the same process is applied. This procedure is continued indefinitely or until $ \beta $ is equal to $ 0 $. The pseudo-code for the continued fraction algorithm is as follows.

loop do
  beta = alpha - alpha.floor
  if beta == 0
    break
  else
    alpha = beta.reciprocal
  end
end

If $ \alpha $ is a rational number, the algorithm is essentially equivalent to the Euclidean algorithm and terminates after finitely many steps. Otherwise, the algorithm is infinite.

The name continued fraction stems from the fact that the quotients in the algorithm can be used to express $ \alpha $ as an “infinite fraction”. In particular, indexing the quotients as $ q_0, q_1, q_2, \dots $ results in $$ \alpha = q_0 + \cfrac{1}{q_1 + \cfrac{1}{q_2 + \cfrac{1}{\ddots}}} . $$

def continued_fraction(root) do
  q0 = sqrt(root).floor
  alpha = (0, 1, 1)
  quotients = []

  while alpha != (q0, 1, 1) do
    a, b, c = alpha
    q = ((a + b * q0) / c).floor
    a = q * c - a
    b = b * c
    c = b**2 * root - (q * c - a)**2
    d = gcd(a, b, c)

    alpha = (a/d, b/d, c/d)
    quotients.push(q)
  end

  return quotients
end

convergents

The convergents of a continued fraction are the rational numbers obtained using finitely many of the quotients. If the quotients are $ [q_0, q_1, q_2, \dots ] $, then the first 3 convergents are $$ q_0, \quad q_0 + \cfrac{1}{q_1}, \quad q_0 + \cfrac{1}{q_1 + \cfrac{1}{q_2}} $$

pell's equation

Finding integer solutions to an equation of the form $ u^2 − R v^2 = 1 $ is known as solving Pell’s equation.

The Indian mathematician Brahmagupta (~600 CE) developed an identity to derive infinitely many solutions of Pell’s equation from a single solution. He was also able to find solutions to some notoriously difficult examples.

Bhaskara II (~1150 CE) extended Brahmagupta’s techniques to develop his “Circle Method”, which was capable of solving any instance of Pell’s equation.

But it wasn’t until 1768 that Lagrange proved that Bhaskara’s method will always terminate after finitely many steps and yield a solution. Lagrange used continued fractions for his proof.

In particular, Lagrange was able to prove that the convergent right before the end of the period of quotients can be used to produce a solution to Pell’s equation.

def continued_fraction(root) do
  q0 = sqrt(root).floor
  alpha = (0, 1, 1)
  quotients = []
  convergents = [(0, 1), (1, 0)]

  while alpha != (q0, 1, 1) do
    a, b, c = alpha
    q = ((a + b * q0) / c).floor
    a = q * c - a
    b = b * c
    c = b**2 * root - (q * c - a)**2
    d = gcd(a, b, c)

    alpha = (a/d, b/d, c/d)

    *rest, prev, curr = convergents
    next_convergent = (q * curr[0] + prev[0], q * curr[1] + prev[1])

    quotients.push(q)
    convergents.push(next_convergent)
  end

  return quotients
end

def multiply(root, alpha, beta) do
  a, b = alpha
  c, d = beta

  return (
    a * c + b * d * root,
    a * d + b * c
  )
end

def power(root, alpha, exponent) do
  if exponent == 0
    return (1, 0)
  elseif exponent == 1
    return alpha
  else
    half_power = power(root, alpha, exponent/2)
    half_power_squared = multiply(root, half_power, half_power)

    if exponent % 2 == 0
      return half_power_squared
    else
      return multiply(root, alpha, half_power_squared)
    end
  end
end

the solution to archimedes' cattle problem

Recall that we need to find a solution to $ u^2 − 4729494 v^2 = 1 $. Using the continued fraction of $ \sqrt{4729494} $ (which has a period of 92) and its convergents, we arrive at a first solution $ (a, b) $ with $$ a = 109931986732829734979866232821433543901088049 $$ $$ b = 50549485234315033074477819735540408986340 . $$ Amthor calculated these integers by hand in 1880.

However, we also need a solution where $ v $ to be divisible by $ 9314 $, which is not the case for $ v = b $. Amthor proved that the $ 2329^{\text{th}} $ power is the first solution that has a $ v $ divisible by $ 9314 $. More explicitly, $ (A, B) $ is a solution with $ B $ divisible by $ 9314 $, where $ (A, B) $ are defined by $$ A + B \sqrt{4729494} = (a + b \sqrt{4729494})^{2329} . $$ These numbers were not possible for Amthor to compute by hand. He used logarithms to show that $ A $ and $ B $ have $ 103,\!273 $ and $ 103,\!270 $ digits, respectively.

Setting $ t = B / 9314 $ and plugging in, we get $$ k = 957 \cdot 4657 t^2 = \dfrac{957 \cdot B^2}{2 \cdot 9314} , $$ which means the total number of cattle under the sun is $$ \text{Total} = 50389082 \cdot k = \dfrac{50389082 \cdot 957 \cdot B^2}{2 \cdot 9314} . $$

The smallest solution to Archimedes' cattle problem is a number that has $ 206,\!545 $ digits!! $$ \text{Total} = 77602714064868182695302328332138866642323224059233 \, \dots \, 05994630144292500354883118973723406626719455081800 $$

epilogue

As mentioned, Amthor was able to find the exact solution by hand in 1880, but was unable to compute the solution. He correctly computed the number of digits and calculated the first four digits to be 7766. The first three of his digits were correct.

In 1965, mathematicians Williams, German, and Zarnke at the University of Waterloo used an IBM 7040 and an IBM 1620 to compute the full solution. It took 7:49 hours. They published their methods in the Mathematics of Computation. The number itself was deposited in the unpublished mathematical tables of the journal.

In 1981, Harry Nelson used a Cray-1 computer at Livermore National Laboratory to compute and verify the solution in about 10 minutes. He published the result with 1/3 font size on 47 pages of the Journal of Recreational Mathematics.

bang bang con west (february 2020)

I gave a 10-minute soap opera version of this as a talk at !!con west in februrary of 2020. At the end of the talk, I used a 2017 13in MacBook Pro to compute the solution in a tenth of a second using some ruby code that I wrote.

• ruby code on github
• video of my !!con west talk