Collatz Conjecture in Python (Time Complexity: O(log n))


The Collatz conjecture in mathematics asks whether repeating two simple arithmetic operations will eventually transform every positive integer into one. It concerns sequences of integers in which each term is obtained from the previous term as follows:
if the previous term is even, the next term is one half of the previous term.
If the previous term is odd, the next term is 3 times the previous term plus 1.
The conjecture is that these sequences always reach 1, no matter which positive integer is chosen to start the sequence.

You are given an integer n and you should print to the console all the values that n takes throughout the Collatz Conjecture.

Example:

Input: n = 3

Output:
3
10
5
16
8
4
2
1

Explanation:
n = 3  => n is odd  => n becomes 3 * 3 + 1 = 10
n = 10 => n is even => n becomes 10 / 2    = 5
n = 5  => n is odd  => n becomes 5 * 3 + 1 = 16
n = 16 => n is even => n becomes 16 / 2    = 8
n = 8  => n is even => n becomes 8 / 2     = 4
n = 4  => n is even => n becomes 4 / 2     = 2
n = 2  => n is even => n becomes 2 / 1     => 1
n = 1 => stop

Understanding the Problem

The core challenge of the Collatz Conjecture is to repeatedly apply a set of rules to transform a number until it reaches 1. The significance of this problem lies in its simplicity and the fact that it remains an unsolved problem in mathematics. The main pitfall is assuming that the sequence will always reach 1, which is the conjecture itself.

Approach

To solve this problem, we need to repeatedly apply the transformation rules until the number becomes 1. A naive approach would involve using a loop to check the number's parity and apply the corresponding transformation. This approach is straightforward but not necessarily optimal in terms of computational complexity.

Naive Solution

The naive solution involves using a while loop to repeatedly apply the transformation rules until the number becomes 1. This approach is simple but can be inefficient for very large numbers.

Optimized Solution

An optimized solution would still use a loop but could include optimizations such as memoization to store previously computed sequences. However, for the scope of this problem, the naive approach is sufficient and easy to understand.

Algorithm

The algorithm can be broken down into the following steps:

  1. Initialize the sequence with the starting number.
  2. While the number is greater than 1, apply the transformation rules.
  3. If the number is even, divide it by 2.
  4. If the number is odd, multiply it by 3 and add 1.
  5. Print each number in the sequence.

Code Implementation

def collatz_conjecture(n):
    # Print the initial number
    print(n)
    # Continue the loop until n becomes 1
    while n > 1:
        if n % 2 == 0:
            # If n is even, divide it by 2
            n = n // 2
        else:
            # If n is odd, multiply by 3 and add 1
            n = 3 * n + 1
        # Print the current value of n
        print(n)

# Example usage
collatz_conjecture(3)

Complexity Analysis

The time complexity of the Collatz Conjecture is not well-defined due to its unsolved nature. However, for practical purposes, the time complexity can be approximated as O(log n) for most numbers. The space complexity is O(1) as we are not using any additional data structures.

Edge Cases

Potential edge cases include:

To test these edge cases, we can run the algorithm with different values of n and verify the output.

Testing

To test the solution comprehensively, we can use a variety of test cases:

# Test cases
collatz_conjecture(1)  # Edge case
collatz_conjecture(3)  # Example case
collatz_conjecture(27) # Known long sequence
collatz_conjecture(1000000) # Large number

Thinking and Problem-Solving Tips

When approaching problems like the Collatz Conjecture, it's important to:

Conclusion

The Collatz Conjecture is a fascinating problem that combines simple arithmetic operations with deep mathematical implications. By understanding and solving this problem, we can improve our problem-solving skills and gain insights into more complex mathematical concepts.

Additional Resources

For further reading and practice problems related to the Collatz Conjecture, consider the following resources: