This post describes the generalization of computing minimum spanning trees on undirected graphs to the setting of directed graphs. In this setting, we refer to edges as ordered pairs rather than unordered pairs, which are called arcs , a graph $G = (V, A)$ now containing vertices $V$ and arcs or directed edges $A$. Suppose some specific vertex $r\in V$ is denoted as the root of $G$.

Definition 1 ($r$ - arborescence) Given a graph $G = (V, A)$ with root $r$, and $r$-arborescence is a set of arcs $A’\subseteq A$ such that for every $v\in V$:

• There is some $(v, v’) \in A’$
• There is a directed path from $v$ to $r$ using only arcs in $A’$.

So, an arborescence contains a directed path from every vertex to the root $r$. A minimum-cost $r$-arborescence is just an $r$-arborescence that has the smallest weight according to a weight function $w : A \to \mathbb{R}$. Note that running a depth-first search from the root on the reversed set of arcs in $G$ makes it easy to check if it contains an $r$-arborescence.

We’d like to find a (fast) algorithm for computing minimum-weight arborescences. The MST problem had lots of nice properties, and it’s natural to ask if some of those extend to this problem as well. In particular, the cut and cycle rules mean one can greedily add and remove certain edges from a graph when constructing a MST; greedy algorithms generally work for the MST problem. For the min-cost $r$-arborescence problem, an analogous greedy building up would add the min-weight arc into the connected component containing $r$ in each round. However, that won’t necessarily result in an optimal arborescence:

graph LR r C --3-->r C --1-->A A -- 4 --> r

since a greedy approach would take $(C, r)$ and then need to take both $(c, a)$ and $(a,r)$ to complete the arborescence, at which point the arc $(a,r)$ would be unnecessary. This example shows more generally that basic greedy approaches to this problem won’t work.

The algorithm that solves this problem is therefore more sophisticated than in the undirected case. Chu-Liu [CL65], Edmonds [Edm67], and Bock [Boc71] discovered this algorithm independently. We will follow Karp’s description of Edmonds’ algorithm.

The “boundary” edges of a vertex is usually denoted by $\partial_v$; the set of edges leaving a vertex is denoted $\partial^+_v$, and the edges entering a vertex is denoted $\partial^-_v$. This notation is overloaded to also denote the set of edges entering or leaving a set of vertices as well.

For a given vertex $v$, we’ll keep track of the weight of the least expensive edge leaving $v$: define $M_v = \min_{a \partial^+_v} w_a$. Define a graph $G’$ which is otherwise identical to $G$ but for a new weight function $w’_a = w_a - M_v$ for every $a \in \partial^+_v$; this guarantees every vertex has an outgoing arc of weight $0$. it’s useful to think of $M_v$ as the weight that must be paid for any arborescence to contain $v$. The next lemma states that this tranformation doesn’t change the problem in any real way.

$T$ is a min-weight $r$-arborescence for $G$ $\Leftrightarrow$ $T$ is a min-weight $r$-arborescence for $G'$.

Given the above lemma, we consider an algorithm that first includes some $0$-weight arc out of each vertex $v$; if this is an arborescence then it must be minimum weight. Otherwise, the resulting graph contains several connected components, each of which has a directed cycle (by a counting argument relating the number of edges and the number of vertices). For any $0$-weight cycle $C$, we will contract $C$ into a single node, removing arcs inside $C$ and replacing any edges $(v, w)$ for $w\in C$ by the cheapest $(v,w’) for$w’\in C$. Call this graph$G’’$. Let$OPT(G)$be the cost of the min-weight$r$-arborescence on$G$. Then$OPT(G') = OPT(G'')$. First, we show$OPT(G') \leq OPT(G'')$. Given an arborescence for$G''$, some vertices correspond to cycles in$G'$. Expand these cycles and drop some$0$-weight edge along each; this is now an arborescence in$G'$with the same weight as$OPT(G'')$. The min-weight arborescence for$G'$can only have smaller weight. Now, we show$OPT(G'') \leq OPT(G')$. Given a min-weight arborescence$T'$for$G'$, contracting$G'$to$G''$still connects all vertices to the root in$G''$. Therefore, this graph still contains a superset of the edges necessary to create an arborescence in$G''$. This gives us an algorithm for computing arborescences: take all$0$-weight edges in$G’$, contract all cycles this creates, then recurse on$G’’$; as$G’’$has strictly fewer vertices and edges than$G’$, this is a well-founded recursion. Each contraction takes$O(m)$time and there are at most$O(n)$of them, so in total this takes$O(mn)$time. It’s possible to implement this algorithm using better datastructures (Tarjan [Tar77] used priority queues to improve the runtime to$O(\min(m \log n, n^2))$time, and Gabow, Galil, Spencer and Tarjan [GGST86] give an algorithm to solve the min-cost arborescence problem in$O(n \log n + m)\$ time.