NOTICE: This website is a permanently available emergency backup for Project Euler. Although you are unable to sign in here, you can use this website to view problems and check answers when the main website is not available. Problem content was last updated Tuesday, 21 September 2021.

About Project Euler

Leonhard Euler (1707-1783)

Where should I start?

That depends on your background. In the Problems table you will be able to see how many people have solved each problem. As a general rule of thumb the more people that have solved it, the easier it is.

I've written my program but should it take days to get to the answer?

Absolutely not! Each problem has been designed according to a "one-minute rule", which means that although it may take several hours to design a successful algorithm with more difficult problems, an efficient implementation will allow a solution to be obtained on a modestly powered computer in less than one minute.

Does it matter if it takes more than one minute to solve?

Of course not, but that should provide the impetus to return to the problem and see how you can improve your approach. But remember that once you've solved a particular problem you will be able to access a thread relating to that problem and it is here that you may be able to pick some tips from others that have solved it.

I solved it by using a search engine, does that matter?

Making use of the internet to research a problem is to be encouraged as there could be hidden treasures of mathematics to be discovered beneath the surface of many of these problems. However, there is a fine line between researching ideas and using the answer you found on another website. If you photocopy a crossword solution then what have you achieved?

I've checked my program ten times now and I keep getting told my answer is wrong! Have you made a mistake?

With newly released problems it is quite possible that a small error may have slipped through the net, or maybe the wording is slightly ambiguous and the problem has not been explained as well as it could. However, when so many people have hit the target and one marksman misses ten times on the run, he/she can hardly shoot his/her own foot and conclude that because the gun is working properly the fault must lie in the target.

Do you have any hints on solving problems?

Read the details of the problem very carefully and make note of any example cases given. Experiment with pencil and paper to get a feel for the ideas behind the problem. If the ideas are new to you, use the internet or books to get some background; the problem should contain clues as to what to look-up. Try writing a program to generate for simple cases and check that your output agrees with the example cases; this will confirm you've understood the problem and are heading in the right direction. Based on this try to extrapolate to estimate the time it will take to get the final answer and if it's going to take significantly more than a minute rethink your strategy.

Is there anywhere else we can discuss and meet other members?

Indeed there is! There is an alternative phpBB forum which has been set up to allow members to engage in general discussions, share ideas relating to problem solving and programming, make suggestions about the website, and ask for clarification on the wording of a particular problem. The link is given below, but please be aware that your account at this website has not been automatically transferred to the alternative forum and you will need to set up a separate account.

Forum Web Address:

Also please note that your posts will be visible to all members and you are requested to be thoughtful in not posting anything that might explicitly give away how to solve a particular problem.

I learned so much solving problem XXX so is it okay to publish my solution elsewhere?

It appears that you have answered your own question. There is nothing quite like that "Aha!" moment when you finally beat a problem which you have been working on for some time. It is often through the best of intentions in wishing to share our insights so that others can enjoy that moment too. Sadly, however, that will not be the case for your readers. Real learning is an active process and seeing how it is done is a long way from experiencing that epiphany of discovery. Please do not deny others what you have so richly valued yourself.

How can I contact Project Euler?

The only way to contact a Project Euler team member at the moment is via the alternative phpBB forum given above. The team visit regularly and are able to deal with most issues quickly and efficiently.