# 9.8 An Unsolved Problem!

Much of the mathematics you have looked at in the course has been used to help people solve problems for centuries. However, there is much more to mathematics than that. Exciting and new developments are being made all the time, and there are many problems that mathematicians have not been able to work out. This section takes a look at one of these problems. [ It was unsolved at the time of writing! ]

In 1742, a Prussian mathematician, Christian Goldbach, made the following conjecture:

All positive even integers bigger than or equal to 4 can be expressed as the sum of two primes. [ A conjecture is a theory that has not yet been proved to be true. ]

## Definitions

The above statement contains a lot of mathematical terminology, so before going further, here are some helpful definitions.

• An integer is a whole number, negative, zero, or positive; for example, , 23, 0, and 281 are all integers.

• An even integer is one that can be divided exactly by two; for example, , 0, 42, 128, or 1002.

• The last digit of an even integer is always 0, 2, 4, 6, or 8. Integers that are not even are odd. [ A lot of encryption processes depend on prime numbers. ]

• A prime number is a whole number greater than 1 that can only be divided exactly by 1 and itself. The first few prime numbers are 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59.

Now, break down Goldbach’s conjecture into steps:

• “All positive even integers bigger than or equal to 4 …” means the numbers 4, 6, 8, 10, 12, and so on.
• “… Can be expressed as the sum of two primes” means that you can choose any of the numbers from the prime number list to add together. For example, 8 = 5 + 3.

## Activity: Sums of Two Primes

Try to express the following numbers as a sum of two primes.

4, 12, 28, 40, 62

For each number, try to find as many combinations of two primes as you can. Do you think Goldbach’s conjecture is true? It can help if you look at the list of prime numbers systematically, starting with 2. Is there a number in the list that you can add to 2 to get the number you want? Then look at 3, and so on. You should be able to find the following combinations:

or

or or

or or

Are you convinced that Goldbach’s conjecture is always true?

Although the conjecture does work for the few numbers we have tried here, that does not necessarily mean that it is going to work for all the even integers greater than or equal to 4. The even integers go on forever, so it is not possible to check them all individually either by hand or computer—some other way of proving the result is needed.

Although higher branches of mathematics have been successful in tackling similar problems, this one is turning out to be particularly tough. So tough in fact that in March 2000, the publishers Faber and Faber offered a \$1 million prize to anyone who managed to prove the conjecture by March 2002. No one did. So even though many great mathematicians have attempted a proof, over 250 years later, it still hasn’t been done. This website [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)] provides a Goldbach's Conjecture calculator if you would like to try out some even numbers for yourself. It will tell you the pair of primes that add up to your number.

You might find that reassuring the next time you get stumped with a mathematics problem—being stuck is a natural state for a mathematician! It’s often the time when great discoveries are made or when the best learning takes place.

9.7.1 Working Out Inequalities

9.9 Patterns in Nature and Elsewhere