Tag Archives: prime numbers

Finding all prime numbers

Till now I just knew only one way of finding all primes i.e. Sieve of Eratosthenes or space optimized version of the same called segmented sieve or time optimized version called Euler’s Sieve.

All of above work on same fundamentals striking out composite numbers from the beginning and what is remaining are the primes.

Recently I came across another way of finding primes the process is called Sieve of Sundaram.

  • You start with all the integers from 2 to n
  • From the above list remove all numbers which are of the form i + j + 2 * i*j

    1<= i <= j

    i + j + 2 *i * j <= N
  • What is remaining is doubled and incremented by 1 to give all the primes except 2. So basically all odd primes.

Example working:

Let us start with (n = 7) i.e. 1, 2, 3, 4, 5, 6, 7
In first step let us keep i = 1 and j = 1, 2, .. so cross out 1 + 1 + 2*1*1 (=4), 1 + 2 + 2 * 2* 1 (=6) take i = 2 then all are greater than 7 so not needed.
Remaining numbers are 1, 2, 3, 5, 7 So from Sieve of Sundaram all odd primes are:
1*2+1, 2*2+1, 3*2+1 = 3, 5, 7 add 2 and you have your result(all the primes in 1 to n).

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