#483 - Repeated Permutation
We define a permutation as an operation that rearranges the order of the elements \(\{1, 2, 3, ..., n\}\). There are \(n!\) such permutations, one of which leaves the elements in their initial order. For \(n = 3\) we have \(3! = 6\) permutations:
- \(P_1 =\) keep the initial order
- \(P_2 =\) exchange the 1st and 2nd elements
- \(P_3 =\) exchange the 1st and 3rd elements
- \(P_4 =\) exchange the 2nd and 3rd elements
- \(P_5 =\) rotate the elements to the right
- \(P_6 =\) rotate the elements to the left
If we select one of these permutations, and we re-apply the same permutation repeatedly, we eventually restore the initial order.
For a permutation \(P_i\), let \(f(P_i)\) be the number of steps required to restore the initial order by applying the permutation \(P_i\) repeatedly.
For \(n = 3\), we obtain:
- \(f(P_1) = 1\) : \((1,2,3) \to (1,2,3)\)
- \(f(P_2) = 2\) : \((1,2,3) \to (2,1,3) \to (1,2,3)\)
- \(f(P_3) = 2\) : \((1,2,3) \to (3,2,1) \to (1,2,3)\)
- \(f(P_4) = 2\) : \((1,2,3) \to (1,3,2) \to (1,2,3)\)
- \(f(P_5) = 3\) : \((1,2,3) \to (3,1,2) \to (2,3,1) \to (1,2,3)\)
- \(f(P_6) = 3\) : \((1,2,3) \to (2,3,1) \to (3,1,2) \to (1,2,3)\)
Let \(g(n)\) be the average value of \(f^2(P_i)\) over all permutations \(P_i\) of length \(n\).
\(g(3) = (1^2 + 2^2 + 2^2 + 2^2 + 3^2 + 3^2)/3! = 31/6 \approx 5.166666667\mathrm e0\)
\(g(5) = 2081/120 \approx 1.734166667\mathrm e1\)
\(g(20) = 12422728886023769167301/2432902008176640000 \approx 5.106136147\mathrm e3\)
Find \(g(350)\) and write the answer in scientific notation rounded to \(10\) significant digits, using a lowercase e to separate mantissa and exponent, as in the examples above.
Problem text © Project Euler, licensed under CC BY-NC-SA 4.0. Original: projecteuler.net/problem=483. Published Sunday, 5th October 2014, 10:00 am. Solved by 343 members at time of mirroring.
Why this is useful
Numerical Computing. Precision, conditioning, and numerical iteration transfer directly to pricing engines and model calibration (Phases 5, 13, 15).
We classify relevance honestly - not every Euler problem is a trading application.
Prerequisites
Lessons that prepare you:
17.1 Python for Quants: NumPy, pandas, and Vectorization · 19.5 Combinatorics: Counting, Binomials, and Inclusion–Exclusion · 19.14 Computational Complexity, Feasibility Estimation, and Proving Algorithms Correct · 19.4 Exact Arithmetic: Big Integers, Rationals, and Floating-Point Traps · 19.11 Integer Partitions and Counting Structures · 2.4 Taylor Series and Local Approximation · 5.1 Floating-Point Arithmetic, Conditioning, and Stability · 7.1 Probability Spaces, Random Variables, and Distributions
Recommended stepping-stone problems: #777 · #744 · #584
Concepts: combinatorics numerical-methods brute-force-reduction
Likely techniques: big-integer precision-control sorting
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Understand the problem
- What exactly is the input to problem 483? Is it a bound (350), a supplied dataset, or a definition you must generate from?
- What is the required output - restate it precisely: a real number quoted to a stated precision, so the whole computation must control rounding error.
- Which objects exactly are in scope, and which are excluded by the wording (strict vs non-strict inequality, 'distinct', 'proper', 'below' vs 'up to')?
- What do the arguments of g(n), g(3) mean, and what is the value's type (count, sum, probability)?
- What are the edge cases: the smallest legal object, zero/one, ties, and the boundary at exactly 350?
- Why is brute force hard HERE specifically? Estimate the number of candidates implied by 350 and the cost of testing one.
- Which numerical-methods fact would, if true, collapse the search - and can you state it as a testable claim before you look for a proof?
Predict & plan (before you code)
- Predict the strategy: in one sentence, what will your solution do? (The classification says numerical-methods / precision-control - do you agree, and why?)
- Predict the complexity of your intended method in terms of N = 350, and the wall-clock time you expect. Write both down now.
- Predict the key data structure: what is stored, keyed by what, and how large will it get at full scale?
- Predict the failure mode: what is most likely to break - an off-by-one on the bound, a definition misread, precision, or memory?
- Predict the output of the small case from rung 3 BEFORE running it - then run it. A surprise here is worth more than an hour of debugging later.
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You have a correct answer. That is the start of the learning, not the end.
- Reduce the time complexity. What is the bottleneck, and what mathematical fact removes it?
- Reduce memory. Can you stream, or keep only the last k states?
- Replace brute force with a closed form, a sieve, a recurrence, or a symmetry argument.
- Prove the optimized version computes the same thing.
- Compare two implementations and time them.
Explain it
Which step of your solution were you least confident about, and what evidence would settle it?
What did you try first, and what specifically made you abandon it - a proof, a timing, or a wrong small-case answer?
Where did the numerical-methods structure do the real work? Name the single observation that collapsed the search space.
Could you have reached the precision-control idea faster? Which words in the statement were pointing at it, and did you notice them?
What was the bug that cost you the most time, and what CLASS of bug was it (off-by-one, definition misread, precision, state under-specified)?
How would your solution change if the bound 350 were multiplied by 1000? Does it survive, or does it need a different idea?
What is the honest complexity of what you wrote (not what you intended), and where is the remaining slack?
Which problem you have already solved is this most similar to, and what is the shared skeleton - is it really 'precision-control' underneath?
State the transferable technique in one sentence, without mentioning this problem's story at all.
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Mastery check
- Variation: change the bound (or a rule) in the statement. Does your method still work? What breaks first?
- Constraints: if the limit were 10× larger, which step fails, and what would you replace it with?
- Related problem: #777 · #744 · #584
- Transfer: where else does this technique appear? Name a lesson and a real computational setting.
- Spaced re-attempt: come back after the review interval and re-solve it with no hints.