What is the biggest number you can think of?

A million?

A billion?

A trillion?

Beyond these familiar terms, mathematicians rely on numbers so huge they are practically useless, but which mathematicians need to test their theories.

**Prof Hugh Woodin, University of California, USA** - *"One of the largest numbers we have a name for is a googol, and it's one followed by a hundred zeros. A hundred zeros is a lot because each zero represents another factor of 10."*

Rather than writing out 100 zeros, scientific notation indicates the number of zeros as a power.

This means only five digits - 10 to the power of 100 - are needed to represent a magnitude greater than the number of atoms in the known Universe.

But the googol is nothing compared to its big brother.

**Prof Peter Cameron, University of London, UK** - *"The googol itself was only a stepping stone on the way to a much, much larger number called a googolplex. A googolplex is 10 raised to the power of a googol; that is, it's one followed by a googol of zeros."*

A googolplex is so large, there is not enough matter in existence to write it longhand.

As numbers increase towards infinity, mathematicians know less and less about them.

But there exists a number that is the biggest ever used.

It is so big that even so-called power towers are useless to describe it.

**Prof Hugh Woodin, University of California, USA **- *"Graham's number is much, much bigger than a googolplex. In fact, it's as large relative to a googolplex as a googolplex is to the number 10. In fact, it's much, much bigger than that!"*

Mathematician Ron Graham discovered his number in 1977, when he was working on a complex equation involving multidimensional cubes.

He had to create an extreme upper limit - the highest value at which his equation would work.

**Limit:Boundary valueCan be high or low**

Graham came up with such a big number it isn't written using powers, but needs a special form of complicated notation.

Because the number is a limit, Graham knows it must be finite.

But he doesn't even know what the first digit is.

Although he has finally managed to calculate the last!

**Prof Ron Graham, University of California, USA** -* "So that means that the remainder when we divide by 10 is always 7 - in other words, if I may conclude, that the last digit of Graham's number is 7. End of the story!"*

Although it does beg the question: what happens if you add one?