Lagmental Vicfred

Bellman--Ford Detects Reachable Negative Cycles by Vicfred

Last updated: Wed 20 November 2024

Relaxing all edges n minus one times finds shortest paths unless another relaxation reveals a negative cycle. The point is to make the formal expression readable enough to audit line by line.

Objects and notation

Weighted paths minimize \(\sum_{e\in P}w(e)\), flows use values \(f:E\to\mathbf R_{\ge0}\) with conservation, and matchings enforce degree at most one. Each problem has a dual obstruction.

$$ d(v)\le d(u)+w(u,v)\qquad((u,v)\in E) $$

The typography mirrors the proof: first declare \(\mathsf D\), then state \(\mathsf C\). The symbol \(\Longrightarrow\) below is a logical dependency, not extra mathematical structure.

$$ \text{a relaxation on pass }|V|\Longrightarrow\text{a reachable negative cycle} $$

Push the symbols

The computation below is not a second theorem. It is a checksum for the definitions and a place to inspect the difficult LaTeX at full size.

$$ \delta_k(v)=\min_{(u,v)\in E}\{\delta_{k-1}(u)+w(u,v)\},\qquad k=1,\ldots,|V|-1 $$

Structural reading

The formula is reusable precisely because it says which pieces are structural and which belong only to the worked example.

$$ \begin{aligned} \mathsf{D}\;&:\quad d(v)\le d(u)+w(u,v)\qquad((u,v)\in E),\\[5pt] \mathsf{C}\;&:\quad \text{a relaxation on pass }|V|\Longrightarrow\text{a reachable negative cycle}. \end{aligned} $$

A hypothesis worth keeping

Dijkstra requires nonnegative edge weights, while flow capacities must be interpreted on a directed residual network even when the original graph is undirected.

$$ \boxed{\begin{gathered} \text{compact conclusion}\\[-2pt] \text{a relaxation on pass }|V|\Longrightarrow\text{a reachable negative cycle} \end{gathered}} $$

A symbolic summary is trustworthy only because the example and limitation remain visible beside it. The box compresses the conclusion without hiding its origin.

This article was posted on Fri 24 April 2020. Facts and circumstances may have changed since publication.
Please contact me before jumping to conclusions if something seems wrong or unclear.