# The Sieve of Sundaram

The Sieve of Eratosthenes is probably the best known algorithm for generating primes. Together with wheel factorization and other optimization options, it can generate primes very quickly.

But a lesser well known algorithm for sieving primes is the Sieve of Sundaram. This algorithm was discovered in 1934 by Sundaram; like the sieve of Eratosthenes it finds all prime numbers up to a certain integer.

### The algorithm

A simplified version of the algorithm, using N as the limit to which we want to find primes to:

m =: Floor of N/2
L =: List of numbers from 1 to m
For every solution (i,j) to i + j + 2ij < m:
Remove i + j + 2ij from L

For each k remaining in L:
2k + 1 is prime.


In practice we can find solutions to $i + j + 2ij < m$ by using two nested for loops:

For i in 0 to m:
For j in i to m:
L[i + j + 2ij] =: False


Here i is always less than j, because the two are interchangeable and filtering it twice would be a waste.

We don’t actually need to loop j from 0 to m. From the inequality $i + j + 2ij < m$, we can solve for j: $j < \frac{m-i}{2i+1}$. The new algorithm:

m =: Floor of N/2
L =: Boolean array of length m
Fill L with true

For i in 0 to m:
For j in i to (m-i)/(2i+1):
L[i + j + 2ij] =: False

For each k remaining in L:
2k + 1 is prime.


### Why this algorithm works

In the algorithm, $2k+1$ is prime where k can be written as $i + j + 2ij$ where i and j are integers. We can rewrite this:

$\begin{array}{l} 2(i+j+2ij) + 1 \\ = 2i + 2j + 4ij + 1 \\ = (2i+1) (2j+1) \end{array}$

Both $2i+1$ and $2j+1$ are odd numbers, and any number that can be written as the product of two odd numbers are composite.

Of the odd numbers, those that cannot be written as the product of two odd numbers are prime. We’ve filtered everything that can be written as $(2i+1)(2j+1)$ so we are left with the odd prime numbers.

This algorithm only gets the odd prime numbers, but fortunately there is only one even prime number, 2.

### Benchmarks with the Sieve of Eratosthenes

Here’s an implementation of the Sieve of Sundaram:

#include <stdio.h>
#include <stdlib.h>

typedef unsigned long long ll;
int main() {
ll n = 100000000LL;
ll m = n/2;
char *sieve = malloc(m);

for(ll i=0; i<m; i++)
sieve[i] = 1;

for(ll i=1; i<m; i++)
for(ll j=i; j<=(m-i)/(2*i+1); j++)
sieve[i+j+2*i*j] = 0;

ll s=1;
for(ll i=1; i<m; i++)
if(sieve[i]) s++;

printf("%llu", s);
return 0;
}


This code counts the number of primes below 100 million, which should be 5761455. The above code runs in 9.725 seconds.

Here’s an alternative, an implementation of the more standard Sieve of Eratosthenes:

#include <stdio.h>
#include <stdlib.h>

typedef unsigned long long ll;
int main(){
ll lim = 100000000LL;
char *sieve = malloc(lim);

for(int i=0; i<lim; i++)
sieve[i] = 1;

int s=0;
for(int i=2; i<lim; i++){
if(sieve[i]){
s++;
for(int j=2; j<=lim/i; j++)
sieve[i*j] = 0;
}
}

printf("%d", s);
return 0;
}


I was surprised to find that the Sieve of Eratosthenes actually ran faster. It completed in 7.289 seconds.

I expected the Sieve of Sundaram to be faster because according to Wikipedia this algorithm uses $O(n \log(n))$ operations, while the Sieve of Eratosthenes uses $O(n \log(n) \log \log(n))$.