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This could be the largest synthetic code dataset yet How to measure the performance of a quantum computer | IBM Quantum Computing Blog Release News: Qiskit v2.5 is here! | IBM Quantum Computing Blog CoFrGeNets replace the ‘bones’ of transformer-based models How training environments can teach AI models to misbehave What’s new at IBM Quantum - Q2 2026 | IBM Quantum Computing Blog Modeling the chemistry of fusion reactor material | IBM Quantum Computing Blog Ponder This Challenge - July 2026 - Return of the Superheroes Apply to IBM Quantum Developer Conference 2026 | IBM Quantum Computing Blog Qiskit Paulice: postselected quantum error correction | IBM Quantum Computing Blog What is IBM’s nanostack chip architecture? IBM introduces the smallest computer chip in the world A new playbook for quantum optimization benchmarking Running AI on mixed hardware for speed and affordability Explore next-gen quantum algorithms with IBM Quantum Credits | IBM Quantum Computing Blog Allstate explores quantum computing for insurance portfolios | IBM Quantum Computing Blog Can LLMs discover quantum error correction codes? Prototype and validate fermionic circuits faster with ffsim | IBM Quantum Computing Blog Bringing the power of semantic AI to IBM Db2 The fast Fourier transform, how and why it works Building AI more like software The future of quantum takes center stage at NY Tech Week Qiskit Fall Fest 2026: Applications open | IBM Quantum Computing Blog IBM to invest $10 billion in quantum computing | IBM Quantum Computing Blog Renowned mathematician Subhash Khot joins IBM Research Ponder This Challenge - June 2026 - The Superhero Team Movies New Classroom Accounts expand quantum access for educators | IBM Quantum Computing Blog Qiskit Global Summer School 2026: Registration now open | IBM Quantum Computing Blog How researchers built a record-setting quantum circuit | IBM Quantum Computing Blog IBM charts a new research path with MIT How IBM is using quantum computing to understand the operating system of the universe How to use sample-based quantum diagonalization on IBM hardware Quantum-centric supercomputing simulates 12,635-atom protein | IBM Quantum Computing Blog A decade of quantum on the cloud | IBM Quantum Computing Blog Ponder This Challenge - May 2026 - The Powers of a Binary Matrix Where the frontiers of high-speed racing and computing meet Introducing the IBM Granite 4.1 family of models Building the future of computing, together Next-generation algorithms could move fusion from the lab to the grid Bringing quantum-centric supercomputing to Illinois What’s new at IBM Quantum - Q1 2026 | IBM Quantum Computing Blog Release News: Qiskit v2.4 is here! | IBM Quantum Computing Blog How IBM Quantum is enabling healthcare and biology research | IBM Quantum Computing Blog How an extra training step can unlock AI’s reasoning power IBM demonstrates extreme scale for content-aware storage with a 100-billion vector database Ponder This Challenge - April 2026 - The Unlabeled Clock IBM Research and ETH Zurich open a new era of innovation IBM’s newest time-series models cover a full range of enterprise prediction tasks Toward a transparent supply chain for AI Quantum computers take a step into real materials science Donating llm-d to the Cloud Native Computing Foundation Cleveland Clinic & IBM debut new quantum simulation workflow | IBM Quantum Computing Blog Turning turbulence into transcripts Like the information in a dream: IBM’s Charles H. Bennett receives ACM Turing award Doubling down on open-access quantum computing | IBM Quantum Computing Blog Unveiling the first reference architecture for quantum-centric supercomputing Realizing Feynman’s vision for the future of simulation | IBM Quantum Computing Blog IBM is working today to secure communication from tomorrow’s quantum risks Building PyTorch-native support for the IBM Spyre Accelerator Quantum simulates properties of the first-ever half-Möbius molecule, designed by IBM and researchers A look back at the International Year of Quantum | IBM Quantum Computing Blog TerraStackAI: Bringing Earth and space AI to Red Hat and the world IBM demonstrates High NA EUV process capability on track for insertion below 2 nm nodes at SPIE 2026 Quantum Advantage Tracker: the race to advantage | IBM Quantum Computing Blog
Ponder This Challenge - March 2026 - Path game on a hole-riddled chessboard
Gadi Aleksandrowicz · 2026-03-01 · via IBM Research

Given an N×MN\times M board and a starting location (x,y)(x,y) with 1xN1\le x\le N and 1yM1\le y\le M, Alice and Bob play the following game: Alice begins by placing a pawn on square (x,y)(x,y), then Bob moves the pawn to an adjacent square of the four potential neighboring squares (diagonals don't count) and removes the square the pawn left from the board. Alice makes the next move, and they alternate until a player has no possible move and loses the game.

Since this is a game obeying Zermalo's theorem, given N×MN\times M and (x,y)(x,y) either Alice can force a win, or Bob can force a win. Call (x,y)(x,y) an "A" square for the board if Alice can force a win, and "B" if Bob can force a win.

To complicate the game, we can remove some of the squares in the board before the game begins, in the following manner. Fix two prime numbers p,qp,q, and label each square in the board with a number, such that square (i,j)(i,j) is labelled pi+qjp^i+q^j (recall that indexing starts from 1, not 0). Now, pick a prime number ss, and remove from the board all the squares whose label is divisible by ss.

For example, it can be seen that for N=M=3N=M=3, the board is

A B A
B A B
A B A

But if we choose p=19,q=2,s=5p=19, q=2, s=5, as 192+22=36519^2+2^2=365 which is divisible by 5 the board becomes

B B B
B # B
B B B

With # denoting the missign square in the middle.

Given a board size N×MN\times M, two primes p,qp,q and a set SS of prime numbers, for each sSs\in S we can count the number of "A" and "B" squares in the board (we don't count "#"). We can then sum those values over all the possible sSs\in S.

For example, for N=M=3N=M=3 and p=19,q=2p=19, q=2 and S=[2,3,5,7,11]S = [2,3,5,7,11] we have a total of 1616 "A" and 1919 "B".

Your goal: Find the total number of "A" and "B" for N=M=157N=M=157, p=419,q=211p=419, q=211 and SS being the set of all prime numbers lower than 100.

A bonus "*" will be given for finding the total number of "A" and "B" for N=M=1557N=M=1557, p=419,q=211p=419, q=211 and SS being the set of all prime numbers lower than 500.

Solution

  • The numerical solutions are:

    • For n=157n=157:
      • Total A: 259501
      • Total B: 280200
    • For n=1557n=1557:
      • Total A: 109008067
      • Total B: 112989820

    Iterating over all cases is simple. The challange in the riddle is - given a specific board, how to find the "A" and "B" squares in it.

    This is an example of the undirected vertex geometry game, which can be played in general on any finite undirected graph, and there is a relatively simple graph-theoretic criterion: A vertex is an "A" vertex (whoever starts on it can force a win) if there exists a maximum matching of the graph that does not contain that vertex.

    Since the graph in question is bipartite, one can use the Hopcroft-Karp algorithm to find one maximum matching. Once such a matching is found, the Dulmage–Mendelsohn decomposition can be used to identify which vertices are part of every maximum matching of the graph.

Solvers

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