#647 - Linear Transformations of Polygonal Numbers
It is possible to find positive integers \(A\) and \(B\) such that given any triangular number, \(T_n\), then \(AT_n +B\) is always a triangular number. We define \(F_3(N)\) to be the sum of \((A+B)\) over all such possible pairs \((A,B)\) with \(\max(A,B)\le N\). For example \(F_3(100) = 184\).
Polygonal numbers are generalisations of triangular numbers. Polygonal numbers with parameter \(k\) we call \(k\)-gonal numbers. The formula for the \(n\)th \(k\)-gonal number is \(\frac 12n\big(n(k-2)+4-k\big)\) where \(n \ge 1\). For example when \(k = 3\) we get \(\frac 12n(n+1)\) the formula for triangular numbers.
The statement above is true for pentagonal, heptagonal and in fact any \(k\)-gonal number with \(k\) odd. For example when \(k=5\) we get the pentagonal numbers and we can find positive integers \(A\) and \(B\) such that given any pentagonal number, \(P_n\), then \(AP_n+B\) is always a pentagonal number. We define \(F_5(N)\) to be the sum of \((A+B)\) over all such possible pairs \((A,B)\) with \(\max(A,B)\le N\).
Similarly we define \(F_k(N)\) for odd \(k\). You are given \(\sum_{k} F_k(10^3) = 14993\) where the sum is over all odd \(k = 3,5,7,\ldots\).
Find \(\sum_{k} F_k(10^{12})\) where the sum is over all odd \(k = 3,5,7,\ldots\)
Problem text © Project Euler, licensed under CC BY-NC-SA 4.0. Original: projecteuler.net/problem=647. Published Sunday, 16th December 2018, 07:00 am. Solved by 536 members at time of mirroring.
Why this is useful
Mathematical Foundation. Exact integer reasoning and algorithmic efficiency. Foundational rigour and computational discipline rather than a direct trading application.
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Prerequisites
Lessons that prepare you:
1.3 Proof Techniques: Direct, Contrapositive, and Contradiction · 19.14 Computational Complexity, Feasibility Estimation, and Proving Algorithms Correct · 19.1 Divisibility, GCD, and the Euclidean Algorithm · 19.3 Modular Arithmetic, Inverses, and Fast Exponentiation · 19.10 Search: Backtracking, Branch-and-Bound, Binary Search, Meet-in-the-Middle · 19.2 Primes, Sieves, and Integer Factorization · 3.1 Vectors, Multivariable Functions, and Level Sets
Recommended stepping-stone problems: #577 · #349 · #745
Concepts: geometry number-theory brute-force-reduction
Learning mode
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Understand the problem
- What exactly is the input to problem 647? Is it a bound (10^12), a supplied dataset, or a definition you must generate from?
- What is the required output - restate it precisely: a single sum.
- 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 constraint does the bound 10^12 impose, and is it inclusive or exclusive?
- What are the edge cases: the smallest legal object, zero/one, ties, and the boundary at exactly 10^12?
- Why is brute force hard HERE specifically? Estimate the number of candidates implied by 10^12 and the cost of testing one.
- Which geometry 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 geometry - do you agree, and why?)
- Predict the complexity of your intended method in terms of N = 10^12, 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 (the statement says: "It is possible to find positive integers A and B such that given any triangular number, T_n, then AT_n +B is always a triangular number.") - then run it. A surprise here is worth more than an hour of debugging later.
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Optimization
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 geometry structure do the real work? Name the single observation that collapsed the search space.
Could you have reached the geometry 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 10^12 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 'geometry' 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: #577 · #349 · #745
- 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.