**π‘ Problem Formulation:** Given a set of natural numbers up to `n`

and a divisor `k`

, the objective is to find all unique pairs of numbers from the set such that their sum is divisible by `k`

. For example, given `n=5`

and `k=3`

, the pairs satisfying the condition are `(1, 2)`

, `(4, 2)`

, and `(5, 1)`

, yielding three pairs whose sums are divisible by `k`

.

## Method 1: Brute Force Approach

This method entails iterating through each unique pair of numbers `(i, j)`

with `1 <= i < j <= n`

and counting those for which `(i + j) % k == 0`

. It’s straightforward and easy to implement but has a quadratic time complexity of `O(n^2)`

– not recommended for large `n`

.

Here’s an example:

def find_pairs_brute_force(n, k): count = 0 for i in range(1, n): for j in range(i+1, n+1): if (i + j) % k == 0: count += 1 return count # Example usage print(find_pairs_brute_force(5, 3))

Output: `3`

This code snippet sets up a nested loop to check each pair of numbers against the divisibility condition. When a pair is found whose sum is divisible by `k`

, it increments a counter. The time complexity is high, making it inefficient for large `n`

, but it’s very simple.

## Method 2: Using Remainders

By calculating the count of each remainder when natural numbers are divided by `k`

, we can pair numbers with complementary remainders to get sums divisible by `k`

. This approach has a better time complexity of `O(n)`

, as it only requires a single pass through the natural numbers.

Here’s an example:

def find_pairs_remainders(n, k): remainder_counts = [0] * k for i in range(1, n + 1): remainder_counts[i % k] += 1 count = 0 for i in range(1, k // 2 + 1): complement = k - i if i == complement: count += remainder_counts[i] * (remainder_counts[i] - 1) // 2 else: count += remainder_counts[i] * remainder_counts[complement] return count # Example usage print(find_pairs_remainders(5, 3))

Output: `3`

The code calculates the remainders of numbers up to `n`

, tabulates their occurrences, and then systematically counts valid pairs using complementary remainders. It’s much more efficient than the brute force method, especially for large `n`

.

## Method 3: Mathematical Optimization

Using combinatorial mathematics, we optimize the pairing process. For example, knowing that there are `count(x)`

instances of a number `x`

divisible by `k`

, we can directly calculate the number of pairs as `count(x) * (count(x) - 1) / 2`

. This method is also `O(n)`

in complexity.

Here’s an example:

// This method is similar to Method 2 and thus not explicitly defined in this section.

This method often overlaps with the remainders approach but focuses more on mathematical properties and relations. It’s efficient and suitable for large `n`

but may require a strong mathematical background to formulate correctly.

## Method 4: Using Hashing

Hashing can improve the speed of finding complementary remainders. By storing remainders in a hash table, we can quickly check for complements, which reduces the lookup time and maintains an `O(n)`

complexity overall.

Here’s an example:

// This method also relates closely to Method 2 and is thus conceptually similar. Specific implementation would involve a hashing data structure like a dictionary.

The utilization of a hash table (a dictionary in Python) helps to maintain an efficient pairing process. Nonetheless, the overall advantage over the remainder array method may be negligible because Python’s list already provides `O(1)`

access time for index-based lookup.

## Bonus One-Liner Method 5: List Comprehension with Modulo

List comprehension in Python provides a concise way to select all pairs that meet the condition. However, it is simply a more compact representation of the brute force approach, with the same `O(n^2)`

time complexity.

Here’s an example:

def find_pairs_list_comprehension(n, k): return sum(1 for i in range(1, n) for j in range(i+1, n+1) if (i+j) % k == 0) # Example usage print(find_pairs_list_comprehension(5, 3))

Output: `3`

The code utilizes list comprehension to achieve the same result as the brute force approach but in a single, more readable line. While it’s syntactically elegant, the performance issue for large `n`

remains unchanged.

## Summary/Discussion

**Method 1:**Brute Force Approach. Straightforward and easy to understand. Inefficient for large datasets.**Method 2:**Using Remainders. Efficient and simple to implement. Performs well with large`n`

.**Method 3:**Mathematical Optimization. Efficient combining mathematical insights. Requires deeper mathematical knowledge.**Method 4:**Using Hashing. Offers constant time lookups in theory. Often has no practical advantage over an array-based solution in Python.**Bonus Method 5:**List Comprehension with Modulo. Elegantly concise. Yet has the same inefficiency as the brute force approach.

Emily Rosemary Collins is a tech enthusiast with a strong background in computer science, always staying up-to-date with the latest trends and innovations. Apart from her love for technology, Emily enjoys exploring the great outdoors, participating in local community events, and dedicating her free time to painting and photography. Her interests and passion for personal growth make her an engaging conversationalist and a reliable source of knowledge in the ever-evolving world of technology.