The 3n+1 Conjecture¶

Author: Franco Saliola <saliola at gmail.com>

The $3n+1$ conjecture is an unsolved conjecture in mathematics. It is also known as the Collatz conjecture, as the Ulam conjecture (after Stanislaw Ulam), or as the Syracuse problem. Lothar Collatz was the first person to propose the problem in 1937.

The 3n+1 operation¶

Consider the following operation on positive integers $n$ .

If $n$ is even, then divide it by $2$ .
If $n$ is odd, then multiply it by $3$ and add $1$ .

For example, if we apply this transformation to $6$ , then we get $3$ since $6$ is even; and if we apply this operation to $11$ , then we get $34$ since $11$ is odd.

Exercises

Write a function that implements this operation, and compute the images of $1, 2, \ldots 100$ .

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Statement of the conjecture¶

If we start with $n=6$ and apply this operation, then we get $3$ .

If we now apply this operation to $3$ , then we get $10$ .

Applying the operation to $10$ outputs $5$ .

Continuing in this way, we get a sequence of integers.

For example, starting with $n=6$ , we get the sequence:

$6, 3, 10, 5, 16, 8, 4, 2, 1, 4, 2, 1, 4, 2, 1, 4, 2, 1, \ldots$

Notice that this sequence has entered the loop $4 \mapsto 2 \mapsto 1 \mapsto 4$ . One formulation of the Collatz conjecture is the following.

3n+1 conjecture: For every positive integer $n$ , the resulting sequence will always reach the number $1$ .*

Exercises

Write a function that takes a positive integer and returns the sequence until it reaches $1$ . For example, for 6 , your function will return [ 6, 3, 10, 5, 16, 8, 4, 2, 1 ]. Find the largest values in the sequences for 1, 3, 6, 9, 16, 27

(Hint : You might find a while helpful here. Below is a very simple example that repeatedly adds 2 to the variable x until x is no longer less than 7.)
```
x = 0
while x < 7:
    x = x + 2
print x
```
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Use the line command to plot the sequence for 27.

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Write an @interact function that takes an integer $n$ and plots the sequence for $n$ .

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Stopping Time¶

The number of steps it takes for a sequence to reach $1$ is the stopping time . For example, the stopping time of $1$ is $0$ and the stopping time of $6$ is $8$ .

Exercises

Write a function that returns the stopping time of a poisitve integer $n$ . Plot the stopping times for $1, 2, ..., 100$ in a bar chart.

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Find the number less than $1000$ with the largest stopping time. What is its stopping time? Repeat this for $2000, 3000, ..., 10000$ .

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Extension to Complex Numbers¶

If $n$ is odd, then $3n+1$ is even. So we can instead consider the $\frac{3n+1}{2}$ -operator that maps $n$ to $\frac{n}{2}$ , if $n$ is even; and to $\frac{3n+1}{2}$ , if $n$ is odd.

Exercises

Implement the $\frac{3n+1}{2}$ -operator.

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Consider the following function.

$f(z)=\frac z 2 \cos^2\left(z \frac \pi 2 \right)+\frac{(3z+1)}{2}\sin^2\left(z \frac \pi 2 \right)$

Construct $f$ as a symbolic function and use Sage to verify that $f(n) = T(n)$ for all $1 \leq n \leq 1000$ , where $T$ is the $\frac{3n+1}{2}$ -operator. Argue that $f$ is a smooth extension of $T$ to the complex plane.

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Let $g(z)$ be the complex function:

$g(z) = \frac{1}{4}(1 + 4z - (1 + 2z)\cos(\pi z))$

Construct $g$ as a symbolic function, and show that $f$ and $g$ are equal. Hint: Explore the various methods for symbolic functions beginning with .trig_.

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Use the complex_plot command to plot g in the domain $x=-5,...,5$ and $y=-5,...,5$ .

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Consider the composition $h_n(z) = (g \circ g \circ \cdots \circ g)$ (where there are $n$ copies of $g$ in this composition). Use complex_plot and graphics_array to plot $h_1$ , $h_2$ , $h_3$ , ..., $h_6$ on the domain $x=1,...,5$ and $y=-0.5,...,0.5$ .

(Hint: To speed things up or control the percision of the computations, you may want to replace pi in your equation with CDF.pi(). Type CDF? and CDF.pi? for more information.)

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Generate some really nice images of $h_n$ that illustrate the fractal-like behaviour of $h_n$ . (Hint: You may want to explore the plot_points and interpolation options for the complex_plot command.)

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