Primes

using System;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static IEnumerable<int> Integers(int from)
    {
        for (;;)
        {
            yield return from;
            from += 1;
        }
    }

    static IEnumerable<int> Helper(int p, IEnumerable<int> xs)
    {
        foreach (var x in xs)
        {
            if (x % p != 0)
                yield return x;
        }
    }

    static IEnumerable<int> Sieve(IEnumerable<int> seq)
    {
        var p = seq.First();
        var xs = seq.Skip(1);

        yield return p;
        foreach (var x in Sieve(Helper(p, xs)))
            yield return x;
    }

    static void Main(string[] args)
    {
        var primes = Sieve(Integers(2));
        foreach (var prime in primes.Take(20))
            Console.WriteLine(prime);
    }
}