Task: Emirp primes
C: #include <stdio.h>

typedef unsigned uint;
int is_prime(uint n)
{
        if (!(n%2) || !(n%3)) return 0;
        uint p = 1;
        while(p*p < n)
                if (n%(p += 4) == 0 || n%(p += 2) == 0)
                        return 0;
        return 1;
}

uint reverse(uint n)
{
        uint r;
        for (r = 0; n; n /= 10)
                r = r*10 + (n%10);
        return r;
}

int is_emirp(uint n)
{
        uint r = reverse(n);
        return r != n && is_prime(n) && is_prime(r);
}

int main(int argc, char **argv)
{
        uint x, c = 0;
        switch(argc) { 
        case 1: for (x = 11; c < 20; x += 2)
                        if (is_emirp(x))
                                printf(" %u", x), ++c;
                break;

        case 2: for (x = 7701; x < 8000; x += 2)
                        if (is_emirp(x))
                                printf(" %u", x);
                break;

        default:
                for (x = 11; ; x += 2)
                        if (is_emirp(x) && ++c == 10000) {
                                printf("%u", x);
                                break;
                        }
        }

        putchar('\n');
        return 0;
}

C#: using static System.Console;
using System;
using System.Linq;
using System.Collections.Generic;

public class Program
{
    public static void Main() {
        const int limit = 1_000_000;
        WriteLine("First 20:");
        WriteLine(FindEmirpPrimes(limit).Take(20).Delimit());
        WriteLine();
		
        WriteLine("Between 7700 and 8000:");
        WriteLine(FindEmirpPrimes(limit).SkipWhile(p => p < 7700).TakeWhile(p => p < 8000).Delimit());
        WriteLine();
		
        WriteLine("10000th:");
        WriteLine(FindEmirpPrimes(limit).ElementAt(9999));
    }
	
    private static IEnumerable<int> FindEmirpPrimes(int limit)
    {
        var primes = Primes(limit).ToHashSet();
		
        foreach (int prime in primes) {
            int reverse = prime.Reverse();
            if (reverse != prime && primes.Contains(reverse)) yield return prime;
	}
    }
	
    private static IEnumerable<int> Primes(int bound) {
        if (bound < 2) yield break;
        yield return 2;
		
        BitArray composite = new BitArray((bound - 1) / 2);
        int limit = ((int)(Math.Sqrt(bound)) - 1) / 2;
        for (int i = 0; i < limit; i++) {
            if (composite[i]) continue;
	    int prime = 2 * i + 3;
	    yield return prime;
			
	    for (int j = (prime * prime - 2) / 2; j < composite.Count; j += prime)
	        composite[j] = true;
        }
	for (int i = limit; i < composite.Count; i++)
	    if (!composite[i]) yield return 2 * i + 3;
    }
}

public static class Extensions
{
    public static HashSet<T> ToHashSet<T>(this IEnumerable<T> source) => new HashSet<T>(source);

    private const string defaultSeparator = " ";
    public static string Delimit<T>(this IEnumerable<T> source, string separator = defaultSeparator) =>
        string.Join(separator ?? defaultSeparator, source);

    public static int Reverse(this int number)
    {
	if (number < 0) return -Reverse(-number);
	if (number < 10) return number;
	int reverse = 0;
	while (number > 0) {
	    reverse = reverse * 10 + number % 10;
	    number /= 10;
	}
	return reverse;
    }
}