Python Program to Find LCM

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The concept of finding the Least Common Multiple (LCM) is a fundamental operation in both arithmetic and number theory, with applications ranging from elementary math problems to advanced computational algorithms. This article aims to serve as a comprehensive guide to writing a Python program for finding the LCM of two or more integers, covering various algorithms, optimization techniques, and best practices.

What is LCM?

The Least Common Multiple (LCM) of two integers aa and bb is the smallest positive integer that is divisible by both aa and bb. For example, the LCM of 4 and 5 is 20 because 20 is the smallest number divisible by both 4 and 5.

Importance of Finding LCM

Finding the LCM is crucial for various reasons:

  • Simplifying fractions
  • Synchronizing events with different periodicities
  • Polynomial arithmetic
  • Computer algorithms in areas like cryptography and signal processing

Basic LCM Algorithm

A straightforward way to find the LCM of two numbers a and b is using the formula:

Here’s a Python function using this approach:

import math

def lcm(a, b):
    return abs(a * b) // math.gcd(a, b)

print(lcm(4, 5))  # Output: 20

Brute-Force Method

A less efficient but simple brute-force method is to increment from the largest of a and b until we find a number that is divisible by both a and b.

def lcm_bruteforce(a, b):
    max_num = max(a, b)
    while True:
        if max_num % a == 0 and max_num % b == 0:
            return max_num
        max_num += 1

print(lcm_bruteforce(4, 5))  # Output: 20

Implementing LCM using GCD

The LCM of two integers aa and bb can also be found using their GCD (Greatest Common Divisor). The relationship between LCM and GCD is given by:

Here’s how you could implement it in python:

def gcd(a, b):
    while b:
        a, b = b, a % b
    return a

def lcm(a, b):
    return abs(a * b) // gcd(a, b)

print(lcm(4, 5))  # Output: 20

Implementing Recursive Method

You can also use recursion to find the LCM, especially if you’re dealing with multiple numbers:

def gcd(a, b):
    if b == 0:
        return a
        return gcd(b, a % b)

def lcm(a, b):
    return abs(a * b) // gcd(a, b)

def lcm_multiple(numbers):
    num_len = len(numbers)
    if num_len == 2:
        return lcm(numbers[0], numbers[1])
        return lcm(numbers[0], lcm_multiple(numbers[1:]))

print(lcm_multiple([4, 5, 10]))  # Output: 20

Performance Considerations

The efficiency of your LCM program depends on the algorithm you choose:

  • The basic LCM algorithm using GCD has a time complexity of O(log ⁡min⁡(a,b)) and is generally sufficient for most applications.
  • The brute-force method has a time complexity of O(a×b) and should generally be avoided due to its inefficiency.


Calculating the Least Common Multiple is a foundational operation in both elementary mathematics and advanced computational algorithms. Although Python doesn’t provide a built-in LCM function, the task can be easily achieved using various methods ranging from leveraging the GCD function to implementing algorithms directly.

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