Some Generalizations of the Bridge and Torch Problem

Pang Ern Thang; Gerard Sayson

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arXiv · 2608.05190

Some Generalizations of the Bridge and Torch Problem

Pang Ern Thang·Gerard Sayson

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Abstract

For the classic bridge and torch problem with crossing times $\left\{1,\ldots,n\right\}$, we derive a closed-form expression for the optimal crossing time $T\left(n\right)$ using a recurrence relation derived from the problem's optimal substructure, thus obtaining \[T\left(n\right)=\frac{n^2}{4}+3n-5+\frac{\left(-1\right)^n-1}{8}\] which holds for all $n\ge 2$. We generalize the problem to a bridge of capacity 3 and obtain the optimal crossing time \[T_3\left(n\right)=\frac{n^{2}}{6}+2n-\frac{181}{36}+\frac{\left(-1\right)^{n}}{4}-\frac{2}{9}\cos\left(\frac{2n\pi}{3}\right)\] which holds for all $n\ge 7$. As such, we obtain a new sequence A392834 in the On-Line Encyclopedia of Integer Sequences. Lastly, we also explore this problem for star graphs, and see how we can recover some classic identities involving the sum of floor functions.

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Pang Ern Thang, Gerard Sayson. 2026-08-02. Some Generalizations of the Bridge and Torch Problem. https://arxiv.org/abs/2608.05190

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