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# How is the time complexity of Sieve of Eratosthenes is n*log(log(n))?

• Difficulty Level : Hard
• Last Updated : 15 Jan, 2020

Pre-requisite: Sieve of Eratosthenes

What is Sieve of Eratosthenes algorithm?
In order to analyze it, let’s take a number n and the task is to print the prime numbers less than n. Therefore, by definition of Sieve of Eratosthenes, for every prime number, it has to check the multiples of the prime and mark it as composite. This process continues until a value p which is the highest prime number less than n.

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Understanding the n*log(log n) time complexity of Sieve of Eratosthenes

1. If it is assumed that the time taken to mark a number as composite is constant, then the number of times the loop runs is equal to:
2. On taking n common from the above equation, the above equation can be rewritten as:
$n*(\frac{1}{2}&space;+&space;\frac{1}{3}&space;+&space;\frac{1}{5}&space;+&space;\frac{1}{7}&space;+&space;......&space;p)$

3. It can be proved as below with the help of Harmonic Progression of the sum of primes:
$\frac{1}{2}&space;+&space;\frac{1}{3}&space;+&space;\frac{1}{5}&space;+&space;\frac{1}{7}&space;+&space;.....&space;=&space;log(log(n))$

Proof of Harmonic Progression of the sum of primes:
In order to understand the proof, the prerequisite is the Harmonic series and Taylor series expansion.

4. On substituting this in the equation, we get the time complexity as:
$O(n*(log(log(n))))$

Hence the time complexity of Sieve of Eratosthenes is n*log(log(n))

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