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arXiv · 2203.11457

On the Frobenius Coin Problem in Three Variables

Negin Bagherpour·Amir Jafari·Amin Najafi Amin
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Abstract

The three-variable Frobenius coin problem asks for the largest integer that cannot be represented as a non-negative integer combination of three positive integers $a_1<a_2<a_3$ with $\gcd(a_1,a_2,a_3)=1$. We introduce an algorithmic framework based on two synchronized Euclidean-division sequences. The method has arithmetic complexity $O(\log a_1)$, matching the best known logarithmic order for the three-variable problem. We prove that this dependence is asymptotically sharp for the proposed algorithm by constructing an explicit infinite Fibonacci-type family for which Step 2 requires $\frac{1}{2}\log_{\phi}(a_1)+O(1)$ iterations. Hence the worst-case arithmetic complexity of the algorithm is $\Theta(\log a_1)$. We also give a constant-overhead reduction from gcd computation to two evaluations of the three-variable Frobenius function. In addition, the synchronized framework yields an upper bound for the Frobenius number, recovers several known special cases, and provides another proof of the Erd\H{o}s--Graham bound. The main advantage of the method is its elementary Euclidean structure, which supports both computation and theoretical analysis.

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Negin Bagherpour, Amir Jafari, Amin Najafi Amin. 2022-03-22. On the Frobenius Coin Problem in Three Variables. https://arxiv.org/abs/2203.11457

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