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An edge is safe for the current accepted forest $F$ if some minimum spanning tree contains both $F$ and that edge. The loop invariant is that some minimum spanning tree contains every accepted edge. It holds initially for the empty edge set. Suppose the next accepted edge $e$ joins a current connected component of a graph $S$ to its complement. No earlier rejected edge crosses this cut of a graph: once its endpoints are in one connected component of a graph, subsequent merges never separate them. Thus $e$ is a lightest edge across that cut of a graph.

For the short exchange argument, take a minimum spanning tree $T$ containing $F$. If $e$ is absent, adding it creates a fundamental cycle. This graph cycle contains another edge $f$ crossing the same cut of a graph. No edge of $F$ crosses the cut of a graph, so $f$ is not in $F$. Replace $f$ by $e$: the result is a spanning tree containing $F$ and $e$, with no greater total weight because $w(e)\le w(f)$. It is therefore also a minimum spanning tree. This is the minimum spanning tree cut property in the form needed for tied weights.

In the example, $CD$ is safe after rejecting $AC$: the current connected component of a graph $\{A,B,C\}$ has crossing edges $CD,CE,BE$ of weights $4,6,7$. Its cheapest connection is $CD$. The rejected $AC$ is also the unique heaviest edge of the graph cycle $A,B,C,A$, so the minimum spanning tree cycle property excludes it from every optimum.

Negative and zero edge weights are allowed: the comparisons and exchange argument still apply. Ties can give several minimum spanning trees, though they need not; any order among equal-weight edges remains valid. Distinct edge weights guarantee a unique minimum spanning tree. To see why, take the lightest edge $e$ belonging to exactly one of two proposed optima, say $T$. Deleting $e$ from $T$ gives a cut of a graph; the other optimum has a graph path across it, containing an edge $f$ absent from $T$. Hence $w(e)<w(f)$, and replacing $f$ by $e$ improves that other optimum, a contradiction. For disconnected input, process all edges to obtain a minimum spanning forest, one minimum spanning tree per original connected component of a graph; no spanning tree of the whole graph exists.^[ref]

A forest containing one minimum spanning tree of each connected component of a graph of a finite undirected weighted graph, including isolated vertices. For disconnected input, Kruskal's algorithm returns a minimum spanning forest.

Among all acyclic graphs contained in a finite weighted undirected graph, with no requirement to connect vertices, one optimum is obtained by keeping only strictly negative edges and computing their minimum spanning forest. Removing a nonnegative edge never worsens weight or creates a cycle. Within a connected component of the negative-edge subgraph, any disconnected candidate forest can be extended by a negative crossing edge, strictly improving its weight. Hence an optimum must span each such component and minimise its tree weight. Kruskal's algorithm on the negative edges therefore solves the problem. Zero-weight edges are optional when they preserve acyclicity.

To find a minimum-weight acyclic subgraph, filter to the subgraph $G_{-}$ containing only strictly negative-weight edges, and run Kruskal's algorithm on those edges, accepting an edge exactly when it joins different current components. Return the resulting forest, with any omitted vertices regarded as isolated. Zero-weight edges may be omitted without worsening the objective.

To prove correctness, first remove all nonnegative edges from any feasible acyclic graph; removing them preserves acyclicity and cannot increase weight. Thus an optimum using only negative edges exists. Inside a connected component of $G_{-}$, any forest not yet connected can be extended by a negative crossing edge: some such edge exists because the original component is connected. Adding it preserves acyclicity and strictly reduces total weight. Consequently an optimum must span each component of $G_{-}$, and its restriction there must be a minimum spanning tree. Kruskal's algorithm finds exactly the resulting minimum spanning forest of $G_{-}$. This proves optimality among all acyclic subgraphs, including disconnected ones.

With $m_{-}$ negative edges, filtering costs $O(|V|+|E|)$, sorting costs $O(m_{-}\log (m_{-}+1))$, and the union-find operations take $O((|V|+m_{-})\alpha (|V|))$ time. A standard overall bound is $O(|V|+|E|\log (|E|+1))$ time. If there are no negative edges, the empty edge set is optimal.

Assume the graph is connected so that a minimum spanning tree exists, and let $T$ be the output of Kruskal's algorithm. With distinct weights there is a strict total edge ordering. Suppose another minimum spanning tree $T^{\prime }\ne T$ existed. Choose $e\in T\setminus T^{\prime }$ and increase only $w(e)$ by a positive $\epsilon$ so small that the strict ordering of all edge weights remains unchanged. For a finite graph such an $\epsilon$ exists, using less than the gap to the next larger weight; if $e$ is largest, any sufficiently small positive increase works.

Running Kruskal's algorithm again examines exactly the same order and makes exactly the same component decisions. It therefore still returns $T$, which must remain optimal for the perturbed weights. But $T$ has become more expensive by $\epsilon$, while $T^{\prime }$ is unchanged. They had equal original weight, so now $T^{\prime }$ is cheaper, a contradiction. This proves that distinct weights imply a unique minimum spanning tree.

The converse is false. A triangle with edge weights $1,1,2$ has a unique minimum spanning tree, consisting of its two weight-one edges. Its total is $2$, while either other spanning tree has total $3$. Thus equal edge weights permit, but do not force, multiple optimum trees. The same perturbation proof applies componentwise to a minimum spanning forest.