They don't have an actual online judge system, all problems simply have a single small string solution, almost always integer or fixed precision floating point, and they just check that you've found the value.
The only metric that matters is who solved the problem first after publication. This is visible e.g. at: projecteuler.net/fastest=454 but only for logged in users... Lol it is ridiculous. The "language" in which problems were solved is just whatever the user put in their profile, they can't actually confirm that.
Once you solve a problem, you can then access its "private" forum thread: projecteuler.net/thread=950 and people will post a bunch of code solutions in there.
projecteuler.net says it started as a subsection in mathschallenge.net, and in 2006 moved to its own domain. WhoisXMLAPI WHOIS history says it was registered by domainmonster.com but details are anonymous. TODO: sample problem on mathschallenge.net on Wayback Machine? Likely wouldn't reveal much anyways though as there is no attribution to problem authors on that site.
www.hackerrank.com/contests/projecteuler/challenges holds challenges with an actual judge and sometimes multiple test cases so just printing the final solution number is not enough.
Project Euler problems typically involve finding or proving and then using a lemma that makes computation of the solution feasible without brute force. There is often an obvious brute force approach, but the pick problem sizes large enough such that it is just not fast enough, but the non-brute-force is.
As such, they live in the intersection of mathematics and computer science.
news.ycombinator.com/item?id=7057408 which is mega high on Google says:
I love project euler, but I've come to the realization that its purpose is to beat programmers soundly about the head and neck with a big math stick. At work last week, we were working on project euler at lunch, and had the one CS PhD in our midst not jumped up and explained the chinese remainder theorem to us, we wouldn't have had a chance.
In many cases, the efficient solution involves dynamic programming.
There are also a set of problems which are very numerical analysis in nature and require the approximation of some real number to a given precision. These are often very fiddly as I doubt most people can prove that their chosen hyperparameters guarantee the required precision.
Many problems ask for solution modulo some number. In general, this is only so that C/C++ users won't have to resort to using an arbitrary-precision arithmetic library and be able to fit everything into uint64 instead. Maybe it also helps the judge system slightly having smaller strings to compare. The final modulos usually don't add any insight to the problems.
Repositories of code solutions:
Repositories with hints but no solutions:
Basically no one has ever had the patience to create a single source that solves them all, all the above were done by individuals and stopped at some point. What we need is either a collaborative solution, or... LLMs.
Using Lean or other programmable proof assistants to solve Project Euler is the inevitable collision of two autisms. In particular, using Lean to prove that you have the correct solution, just making a Lean program that prints out the correct solution is likely now trivial as of 2025 by asking an LLM to port a Python solution to the new language.
Some efforts:
Mentions:
In other proof assistants, therefore with similar beauty:
The beauty of Project Euler is that it would serve both as a AI code generation benchmark and as an AI Math benchmark!
TODO: real name? Occupation?
Claude says he's from the UK and has a background in mathematics. Oxbridge feels likely. How I Failed, Failed, and Finally Succeeded at Learning How to Code says he started off on the ORIC computer, which is British-made, so he is likely British.
This was a registration CAPTCHA problem as of 2025:
Among the first 510 thousand square numbers, what is the sum of all the odd squares?
Python solution:
s = 0
for i in range(1, 510001, 2):
    s += i*i
print(s)
At: euler/0.py
Solution:
233168
The original can be found with:
printf '1\n1000\n' | euler/1.py
A(x) = x + 1
Z(u)(v) = v
S(u)(v)(w) = v(u(v)(w))
Let's resolve the second example ourselves:
S
  (S)
  (S(S))
  (S(Z))
(A)
(0)

S
(S)
(
  S
  (S(S))
  (S(Z))
)
(A)
(0)

S
(S(S))
(S(Z))
(
  S
  (
    S
    (S(S))
    (S(Z))
  )
  (A)
)
(0)

S
(Z)
(
  S(S)
  (S(Z))
  (
    S
    (
      S
      (S(S))
      (S(Z))
    )
    (A)
  )
)
(0)

S(S)
(S(Z))
(
  S
  (
    S
    (S(S))
    (S(Z))
  )
  (A)
)
(
  Z
  (
    S(S)
    (S(Z))
    (
      S
      (
        S
        (S(S))
        (S(Z))
      )
      (A)
    )
  )
  (0)
)

S
(S)
(S(Z))
(
  S
  (
    S
    (S(S))
    (S(Z))
  )
  (A)
)
(0)
TODO: how long would this be?
So we see that all of these rules resolve quite quickly and do not go into each other. S however offers some problems, in that:
C_0 = Z
C_i = S(C_{i-1})
D_i = C_i(S)(S)
So we see that D_i goes somewhat simply into C_i, and C_i is recursive giving:
S^i(Z)
Calculate the nine first digits of:
D_a(D_b)(D_c)(C_d)(A)(e)
Removing D_a:
S^i(Z)S)(S)(D_b)(D_c)(C_d)(A)(e)
Numerical solution:
1038733707
Programs:
A naive T in Python is:
from collections import deque

def T(a: int, b: int, N: int) -> int:
    total = a
    q = deque([a] * (a - 1))
    is_a = False
    for i in range(N - 1):
        cur = q.popleft()
        total += cur
        q.extend([a if is_a else b] * cur)
        is_a = not is_a
    return total

assert T(2, 3, 10) == 25
assert T(4, 2, 10**4) == 30004
assert T(5, 8, 10**6) == 6499871
which passes the tests, but takes half a second on PyPy. So clearly it is not going to work for 22332223332233 which has 14 digits.
Maybe if T is optimized enough, then we can just bruteforce over the ~40k possible sum ranges 2 to 223. 1 second would mean 14 hours to do them all, so bruteforce but doable. Otherwise another optimization step may be needed at that level as well: one wonders if multiple sums can be factored out, or if the modularity can of the answer can help in a meaningful way. The first solver was ecnerwala using C/C++ in 1 hour, so there must be another insight missing, unless they have access to a supercomputer.
The first idea that comes to mind to try and optimize T is that this is a dynamic programming, but then the question is what is the recurrence relation.
The sequence appears to be a generalization of the Kolakoski sequence but to numbers other than 1 and 2, also known as the Generalized Kolakoski sequence.
maths-people.anu.edu.au/~brent/pd/Kolakoski-ACCMCC.pdf "A fast algorithm for the Kolakoski sequence" might provide the solution, the paper says:
It is conjectured that the algorithm runs in time and space , where
and provides exactly a recurrence relation and a dynamic programming approach.
www.reddit.com/r/algorithms/comments/8cv3se/kolakoski_sequence/ asks for an implementation but no one gave anything. Dupe question: math.stackexchange.com/questions/2740997/kolakoski-sequence contains an answer with Python and Rust code but just for the original 1,2 case.
github.com/runbobby/Kolakoski has some C++ code but it is not well documented so it's not immediately easy to understand what it actually does. It does appear to consider the m n case however.
Bibligraphy:
Numerical solution:
1033654680825334184
Numerical solution:
726010935
This problem took several months to get publicly leaked, one of the longest by far on github.com/lucky-bai/projecteuler-solutions
Numerical solution:
176907658
Numerical solution:
736463823
Numerical solution:
6795261671274
Numerical solution:
882086212
Numerical solution:
367554579311
Programs: TODO!
matharena.ai/euler/ mentions that MathArena guys managed to solve it with GPT-5 and an agent.
Numerical solution:
367554579311
Numerical solution:
243559751
Numerical solution:
166666666689036288
Numerical solution:
7259046
Numerical solution:
55129975871328418
Numerical solution:
29337152.09
Programs:
Numerical solution:
357591131712034236
Numerical solution:
885362394
Numerical solution:
44754029
Numerical solution:
33626723890930
Numerical solution:
3575508
Numerical solution:
427278142
Numerical solution:
13313751171933973557517973175
As mentioned at euler.stephan-brumme.com these tend to be harder, as they have their own judge system that actually runs programs, and therefore can test input multiple test cases against their reference implementation rather than just hard testing the result for a single input.
Goes only up to Project Euler problem 254 as of 2025, which had been published much much earlier, in 2009, so presumably they've stopped there.

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Project Euler is a collection of challenging mathematical and computational problems that require creative problem-solving and programming skills to solve. It was started by Colin Hughes in 2001 and is named after the famous mathematician Leonhard Euler. The problems range in difficulty, and they often require a combination of mathematical insight and coding proficiency to derive efficient solutions. The problems typically involve numerical computations, algorithms, and sometimes require knowledge of number theory, combinatorics, or other mathematical areas.
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