I saw this sheet floating around the internet. Very useful formula reference sheet. 

Here is some code for finding factorials using the GMP library.

Please note:

-Really large numbers may take more time than you/your computer are willing/able to spend and that this is not multiple threads so don't go trying to break any records.

 -Compile it with the tags it requires and you need to install libgmp3-dev

//When compiling include -lgmpxx and -lgmp and have sudo apt-get install libgmp3-dev
#include <iostream>
#include <gmpxx.h>
#include <fstream>
using namespace std;
int main()
{
	ofstream fout;
	fout.open("Yournumber.txt");
	mpz_class Number, Temp1, ans, anstwo;
	cout<<"Factorial Calculator\nEnter number: ";
	cin>>Number;

	if (Number==0){ans=0;}
	
	else{ans=1;}

	Temp1=Number;
	for (int i=0;i<Number;i++)
	{
		ans=ans*Temp1;
		Temp1=Temp1-1;
	}
	anstwo=ans;
	for (int j=0;j<11;j++)
	{
		anstwo=ans*anstwo;
	}

	fout<<anstwo;
	cout<<"Answer in Yournumber.txt"<<endl;
	fout.close();
	return 0;
}

Example of using the GMP library to find Fibonacci numbers. 

//When compiling include -lgmpxx and -lgmp and have sudo apt-get install libgmp3-dev
#include <iostream>
#include <fstream>
#include <gmpxx.h>
using namespace std;
int main ()
{
	ofstream fout;
	fout.open("fibonacci_numbers.txt");
	int amount, ans=0, temp1=1, temp2; 
        cout<<"How many?";
        cin>>amount;
	for (int i=0;i<amount;i++)
	{
		fout<<ans<<endl;
		temp2=ans;
		ans=ans+temp1;
		temp1 = temp2;
	}
}